fix(status): suppress false-positive "base merged" when branch hasn't diverged

The ancestor check in detectBranchMerged trivially passes when the branch
and base resolve to the same commit (e.g. a base workspace created from main
with no unique commits yet). Add a same-commit guard after the Phase 1
ancestor check to return null instead of { kind: "merge" }.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: henrikje

src/lib/analysis/merge-detection.test.ts

@@ -125,13 +125,44 @@ describe("detectBranchMerged", () => {
       expect(result).toBeNull();
     }));
 
-  test("returns merge for empty branch (no commits ahead of base)", () =>
+  test("returns null for branch at same commit as base (no divergence)", () =>
     withRepo(async ({ repoDir }) => {
       const defaultBranch = (await getDefaultBranch(repoDir, "origin")) ?? "main";
 
+      // Branch created from default with no additional commits — same SHA as default.
+      // This is NOT a merge; the branch simply hasn't diverged yet.
       Bun.spawnSync(["git", "-C", repoDir, "checkout", "-b", "feature"]);
 
       const result = await detectBranchMerged(repoDir, defaultBranch);
+      expect(result).toBeNull();
+    }));
+
+  test("returns null for branch at same commit as base via explicit branchRef", () =>
+    withRepo(async ({ repoDir }) => {
+      const defaultBranch = (await getDefaultBranch(repoDir, "origin")) ?? "main";
+
+      // Same scenario but using an explicit branchRef (like the stacked base detection does)
+      Bun.spawnSync(["git", "-C", repoDir, "checkout", "-b", "feature"]);
+      Bun.spawnSync(["git", "-C", repoDir, "checkout", defaultBranch]);
+
+      const result = await detectBranchMerged(repoDir, defaultBranch, 200, "feature");
+      expect(result).toBeNull();
+    }));
+
+  test("returns merge for branch that is strictly behind base (ancestor but different commit)", () =>
+    withRepo(async ({ repoDir }) => {
+      const defaultBranch = (await getDefaultBranch(repoDir, "origin")) ?? "main";
+
+      // Create branch, then advance default past it — branch is ancestor of default
+      // but at a different commit. This is the legitimate "branch was merged" or
+      // "branch fell behind" case.
+      Bun.spawnSync(["git", "-C", repoDir, "checkout", "-b", "feature"]);
+      Bun.spawnSync(["git", "-C", repoDir, "checkout", defaultBranch]);
+      writeFileSync(join(repoDir, "main-work.txt"), "main work");
+      Bun.spawnSync(["git", "-C", repoDir, "add", "main-work.txt"]);
+      Bun.spawnSync(["git", "-C", repoDir, "commit", "-m", "advance main"]);
+
+      const result = await detectBranchMerged(repoDir, defaultBranch, 200, "feature");
       expect(result?.kind).toBe("merge");
     }));
 

src/lib/analysis/merge-detection.ts

@@ -53,7 +53,19 @@ export async function detectBranchMerged(
 ): Promise<MergeDetectionResult | null> {
   // Phase 1: Ancestor check (instant) — detects merge commits and fast-forwards
   const ancestor = await gitLocal(repoDir, "merge-base", "--is-ancestor", branchRef, baseBranchRef);
-  if (ancestor.exitCode === 0) return { kind: "merge" };
+  if (ancestor.exitCode === 0) {
+    // Guard: if both refs resolve to the same commit, the branch hasn't diverged —
+    // it's "equal" to the base, not "merged" into it. This avoids false positives
+    // for branches that were created from the base but never had unique commits.
+    const [branchSha, baseSha] = await Promise.all([
+      gitLocal(repoDir, "rev-parse", branchRef),
+      gitLocal(repoDir, "rev-parse", baseBranchRef),
+    ]);
+    if (branchSha.exitCode === 0 && baseSha.exitCode === 0 && branchSha.stdout.trim() === baseSha.stdout.trim()) {
+      return null;
+    }
+    return { kind: "merge" };
+  }
 
   // Resolve base branch patch-id map (shared across Phase 2 + Phase 3, and across workspaces)
   const basePatchIdMap = await resolveBasePatchIdMap(repoDir, baseBranchRef, commitLimit, basePatchIdCache);