status: add status.compareBranches config for multiple branch comparisons

Add a new configuration variable status.compareBranches that allows
users to specify a space-separated list of branch comparisons in
git status output.

Supported values:
- @{upstream} for the current branch's upstream tracking branch
- @{push} for the current branch's push destination

Any other value is ignored and a warning is shown.

When not configured, the default behavior is equivalent to setting
`status.compareBranches = @{upstream}`, preserving backward
compatibility.

The advice messages shown are context-aware:
- "git pull" advice is shown only when comparing against @{upstream}
- "git push" advice is shown only when comparing against @{push}
- Divergence advice is shown for upstream branch comparisons

This is useful for triangular workflows where the upstream tracking
branch differs from the push destination, allowing users to see their
status relative to both branches at once.

Example configuration:
    [status]
        compareBranches = @{upstream} @{push}

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>

Author: HaraldNordgren

Documentation/config/status.adoc

@@ -17,6 +17,25 @@ status.aheadBehind::
 	`--no-ahead-behind` by default in linkgit:git-status[1] for
 	non-porcelain status formats.  Defaults to true.
 
+status.compareBranches::
+	A space-separated list of branch comparison specifiers to use in
+	linkgit:git-status[1]. Currently, only `@{upstream}` and `@{push}`
+	are supported. They are interpreted as `branch@{upstream}` and
+	`branch@{push}` for the current branch.
++
+If not set, the default behavior is equivalent to `@{upstream}`, which
+compares against the configured upstream tracking branch.
++
+Example:
++
+----
+[status]
+	compareBranches = @{upstream} @{push}
+----
++
+This would show comparisons against both the configured upstream and push
+tracking branches for the current branch.
+
 status.displayCommentPrefix::
 	If set to true, linkgit:git-status[1] will insert a comment
 	prefix before each output line (starting with

remote.c

@@ -29,6 +29,12 @@
 
 enum map_direction { FROM_SRC, FROM_DST };
 
+enum {
+	ENABLE_ADVICE_PULL       = (1 << 0),
+	ENABLE_ADVICE_PUSH       = (1 << 1),
+	ENABLE_ADVICE_DIVERGENCE = (1 << 2),
+};
+
 struct counted_string {
 	size_t len;
 	const char *s;
@@ -2234,13 +2240,42 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs,
 	return stat_branch_pair(branch->refname, base, num_ours, num_theirs, abf);
 }
 
+static char *resolve_compare_branch(struct branch *branch, const char *name)
+{
+	const char *resolved = NULL;
+
+	if (!branch || !name)
+		return NULL;
+
+	if (!strcasecmp(name, "@{upstream}"))
+		resolved = branch_get_upstream(branch, NULL);
+	else if (!strcasecmp(name, "@{push}"))
+		resolved = branch_get_push(branch, NULL);
+	else {
+		warning(_("ignoring value '%s' for status.compareBranches; only @{upstream} and @{push} are supported"),
+			name);
+		return NULL;
+	}
+
+	if (resolved)
+		return xstrdup(resolved);
+	return NULL;
+}
+
 static void format_branch_comparison(struct strbuf *sb,
 				     bool up_to_date,
 				     int ours, int theirs,
 				     const char *branch_name,
 				     enum ahead_behind_flags abf,
-				     bool show_divergence_advice)
+				     unsigned flags)
 {
+	bool enable_push_advice = (flags & ENABLE_ADVICE_PUSH) &&
+		advice_enabled(ADVICE_STATUS_HINTS);
+	bool enable_pull_advice = (flags & ENABLE_ADVICE_PULL) &&
+		advice_enabled(ADVICE_STATUS_HINTS);
+	bool enable_divergence_advice = (flags & ENABLE_ADVICE_DIVERGENCE) &&
+		advice_enabled(ADVICE_STATUS_HINTS);
+
 	if (up_to_date) {
 		strbuf_addf(sb,
 			_("Your branch is up to date with '%s'.\n"),
@@ -2249,7 +2284,7 @@ static void format_branch_comparison(struct strbuf *sb,
 		strbuf_addf(sb,
 			    _("Your branch and '%s' refer to different commits.\n"),
 			    branch_name);
-		if (advice_enabled(ADVICE_STATUS_HINTS))
+		if (enable_push_advice)
 			strbuf_addf(sb, _("  (use \"%s\" for details)\n"),
 				    "git status --ahead-behind");
 	} else if (!theirs) {
@@ -2258,7 +2293,7 @@ static void format_branch_comparison(struct strbuf *sb,
 			   "Your branch is ahead of '%s' by %d commits.\n",
 			   ours),
 			branch_name, ours);
-		if (advice_enabled(ADVICE_STATUS_HINTS))
+		if (enable_push_advice)
 			strbuf_addstr(sb,
 				_("  (use \"git push\" to publish your local commits)\n"));
 	} else if (!ours) {
@@ -2269,7 +2304,7 @@ static void format_branch_comparison(struct strbuf *sb,
 			       "and can be fast-forwarded.\n",
 			   theirs),
 			branch_name, theirs);
-		if (advice_enabled(ADVICE_STATUS_HINTS))
+		if (enable_pull_advice)
 			strbuf_addstr(sb,
 				_("  (use \"git pull\" to update your local branch)\n"));
 	} else {
@@ -2282,8 +2317,7 @@ static void format_branch_comparison(struct strbuf *sb,
 			       "respectively.\n",
 			   ours + theirs),
 			branch_name, ours, theirs);
-		if (show_divergence_advice &&
-		    advice_enabled(ADVICE_STATUS_HINTS))
+		if (enable_divergence_advice)
 			strbuf_addstr(sb,
 				_("  (use \"git pull\" if you want to integrate the remote branch with yours)\n"));
 	}
@@ -2296,34 +2330,92 @@ int format_tracking_info(struct branch *branch, struct strbuf *sb,
 			 enum ahead_behind_flags abf,
 			 int show_divergence_advice)
 {
-	int ours, theirs, cmp_fetch;
-	const char *full_base;
-	char *base;
-	int upstream_is_gone = 0;
+	char *compare_branches = NULL;
+	struct string_list branches = STRING_LIST_INIT_DUP;
+	struct strset processed_refs = STRSET_INIT;
+	int reported = 0;
+	size_t i;
+	const char *upstream_ref;
+	const char *push_ref;
 
-	cmp_fetch = stat_tracking_info(branch, &ours, &theirs, &full_base, 0, abf);
-	if (cmp_fetch < 0) {
-		if (!full_base)
-			return 0;
-		upstream_is_gone = 1;
+	repo_config_get_string(the_repository, "status.comparebranches",
+			       &compare_branches);
+
+	if (compare_branches) {
+		string_list_split(&branches, compare_branches, " ", -1);
+		string_list_remove_empty_items(&branches, 0);
+	} else {
+		string_list_append(&branches, "@{upstream}");
 	}
 
-	base = refs_shorten_unambiguous_ref(get_main_ref_store(the_repository),
-					    full_base, 0);
+	upstream_ref = branch_get_upstream(branch, NULL);
+	push_ref = branch_get_push(branch, NULL);
 
-	if (upstream_is_gone) {
-		strbuf_addf(sb,
-			_("Your branch is based on '%s', but the upstream is gone.\n"),
-			base);
-		if (advice_enabled(ADVICE_STATUS_HINTS))
-			strbuf_addstr(sb,
-				_("  (use \"git branch --unset-upstream\" to fixup)\n"));
-	} else {
-		format_branch_comparison(sb, !cmp_fetch, ours, theirs, base, abf, show_divergence_advice);
+	for (i = 0; i < branches.nr; i++) {
+		char *full_ref;
+		char *short_ref;
+		int ours, theirs, cmp;
+		int is_upstream, is_push;
+		unsigned flags = 0;
+
+		full_ref = resolve_compare_branch(branch,
+						  branches.items[i].string);
+		if (!full_ref)
+			continue;
+
+		if (!strset_add(&processed_refs, full_ref)) {
+			free(full_ref);
+			continue;
+		}
+
+		short_ref = refs_shorten_unambiguous_ref(
+			get_main_ref_store(the_repository), full_ref, 0);
+
+		is_upstream = upstream_ref && !strcmp(full_ref, upstream_ref);
+		is_push = push_ref && !strcmp(full_ref, push_ref);
+
+		if (is_upstream && (!push_ref || !strcmp(upstream_ref, push_ref)))
+			is_push = 1;
+
+		cmp = stat_branch_pair(branch->refname, full_ref,
+				       &ours, &theirs, abf);
+
+		if (cmp < 0) {
+			if (is_upstream) {
+				strbuf_addf(sb,
+					_("Your branch is based on '%s', but the upstream is gone.\n"),
+					short_ref);
+				if (advice_enabled(ADVICE_STATUS_HINTS))
+					strbuf_addstr(sb,
+						_("  (use \"git branch --unset-upstream\" to fixup)\n"));
+				reported = 1;
+			}
+			free(full_ref);
+			free(short_ref);
+			continue;
+		}
+
+		if (reported)
+			strbuf_addstr(sb, "\n");
+
+		if (is_upstream)
+			flags |= ENABLE_ADVICE_PULL;
+		if (is_push)
+			flags |= ENABLE_ADVICE_PUSH;
+		if (show_divergence_advice && is_upstream)
+			flags |= ENABLE_ADVICE_DIVERGENCE;
+		format_branch_comparison(sb, !cmp, ours, theirs, short_ref,
+					 abf, flags);
+		reported = 1;
+
+		free(full_ref);
+		free(short_ref);
 	}
 
-	free(base);
-	return 1;
+	string_list_clear(&branches, 0);
+	strset_clear(&processed_refs);
+	free(compare_branches);
+	return reported;
 }
 
 static int one_local_ref(const struct reference *ref, void *cb_data)