cli:converge: new jj converge command

`jj converge` allows users to "fix" divergence. The command tries to
create a new commit for the divergent change that rewrites all divergent
commits. The command tries to do this automatically, but falls back to
prompting the user for pieces of information as needed.

The command uses the lib/converge.rs library to do most of the work.

The command takes an optional --search_space revset (it looks for
divergent commits matching that revset). If not specified the command
uses a new `revsets.converge` system revset (mutable() & divergent()).

If the command cannot automatically merge the descriptions, the user's
text editor is invoked to let the user merge the divergent descriptions
manually (as if they were conflicts on a "description" file).

The command has a --interactive=true/false flag to allow users to invoke
it without prompting the user.

Author: drieber

CHANGELOG.md

@@ -24,6 +24,12 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   jj workspace can have its own Git HEAD. Existing repositories are migrated
   automatically.
 
+* The new `jj converge` command attempts to automatically resolve divergence by
+  creating a new commit that replaces the divergent commits. It applies
+  heuristics to try to automatically come up with a good solution, and falls
+  back to prompting the user if the heuristics are inconclusive. It can also run
+  in non-interactive mode, which aborts if prompting would be needed.
+
 ### Fixed bugs
 
 * The default pager flags now include `-K` (`--quit-on-intr`), so pressing

cli/src/command_error.rs

@@ -28,6 +28,7 @@ use jj_lib::config::ConfigFileSaveError;
 use jj_lib::config::ConfigGetError;
 use jj_lib::config::ConfigLoadError;
 use jj_lib::config::ConfigMigrateError;
+use jj_lib::converge::ConvergeError;
 use jj_lib::dsl_util::Diagnostics;
 use jj_lib::evolution::WalkPredecessorsError;
 use jj_lib::fileset::FilePatternParseError;
@@ -427,6 +428,19 @@ impl From<WalkPredecessorsError> for CommandError {
     }
 }
 
+impl From<ConvergeError> for CommandError {
+    fn from(err: ConvergeError) -> Self {
+        match err {
+            ConvergeError::Backend(err) => err.into(),
+            ConvergeError::Index(err) => err.into(),
+            ConvergeError::RevsetEvaluation(err) => err.into(),
+            ConvergeError::WalkPredecessors(err) => err.into(),
+            ConvergeError::IO(err) => err.into(),
+            ConvergeError::Other(err) => internal_error(err),
+        }
+    }
+}
+
 impl From<DiffEditError> for CommandError {
     fn from(err: DiffEditError) -> Self {
         user_error_with_message("Failed to edit diff", err)
@@ -507,6 +521,12 @@ impl From<TempTextEditError> for CommandError {
     }
 }
 
+impl From<TempTextEditError> for ConvergeError {
+    fn from(err: TempTextEditError) -> Self {
+        Self::Other(Box::new(err))
+    }
+}
+
 impl From<TrailerParseError> for CommandError {
     fn from(err: TrailerParseError) -> Self {
         user_error(err)
@@ -936,7 +956,9 @@ fn revset_resolution_error_hints(err: &RevsetResolutionError) -> Vec<String> {
             kind: _,
             symbol: _,
             targets,
-        } => vec![multiple_targets_hint(targets)],
+        } => {
+            vec![multiple_targets_hint(targets)]
+        }
         RevsetResolutionError::EmptyString
         | RevsetResolutionError::WorkspaceMissingWorkingCopy { .. }
         | RevsetResolutionError::AmbiguousCommitIdPrefix(_)