Enhance branch-cleanup with multi-layered merge detection

Improves the git:branch-cleanup command to detect merged branches using
multiple methods, preventing false negatives when branches are rebased,
squashed, or cherry-picked.

Key improvements:
- Added check_if_merged() function with 4 detection methods:
  * Standard: git branch --merged (commits in main history)
  * Merge-commit: searches merge commit messages
  * Content-identical: compares content with git diff (catches rebases)
  * Cherry-picked: detects equivalent patches with git cherry

- Enhanced categorization to show detection method used
- Updated examples to demonstrate new merge status labels
- Improved output format with explanatory notes for rebased branches

This fixes cases where branches show as unmerged despite having identical
content in main, such as when maintainers rebase PRs before merging.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: mtnbikenc

plugins/git/commands/branch-cleanup.md

@@ -34,12 +34,20 @@ The command should follow these steps:
 2. **Gather Branch Information**
    - List all local branches: `git branch`
    - Get current branch: `git branch --show-current`
-   - Identify merged branches: `git branch --merged <main-branch>`
+   - Identify merged branches using multi-layered detection:
+     - Standard merge: `git branch --merged <main-branch>`
+     - Merge commit messages: `git log <main-branch> --merges --oneline`
+     - Content comparison: `git diff <main-branch>...<branch> --quiet`
+     - Cherry-pick detection: `git cherry <main-branch> <branch>`
    - Check remote tracking: `git branch -vv`
    - Find remote-deleted branches: `git remote prune origin --dry-run`
 
 3. **Categorize Branches**
-   - **Merged branches**: Fully merged into main branch
+   - **Merged branches**: Detected via one of these methods:
+     - Standard: Commits directly in main branch history
+     - Merge commit: Found in main's merge commit messages
+     - Content-identical: All changes present in main (rebased/cherry-picked)
+     - Cherry-picked: All commits have equivalents in main
    - **Gone branches**: Remote tracking branch no longer exists
    - **Stale branches**: Last commit older than threshold (e.g., 3 months)
    - **Protected branches**: main, master, develop, release/*, hotfix/*
@@ -79,8 +87,53 @@ fi
 # Get current branch
 current_branch=$(git branch --show-current)
 
-# Find merged branches
-git branch --merged "$main_branch" | grep -v "^\*" | grep -v "$main_branch"
+# Multi-layered merge detection function
+# Returns: "merged:<method>" or "not-merged"
+check_if_merged() {
+    local branch=$1
+    local main_branch=$2
+
+    # Method 1: Standard merge check (commits in main history)
+    if git branch --merged "$main_branch" | grep -q "^[* ]*${branch}$"; then
+        echo "merged:standard"
+        return 0
+    fi
+
+    # Method 2: Check merge commit messages (catches squash merges and rebased branches)
+    if git log "$main_branch" --merges --oneline | grep -q "Merge branch '${branch}'"; then
+        echo "merged:merge-commit"
+        return 0
+    fi
+
+    # Method 3: Content comparison (handles rebased/cherry-picked branches)
+    # If diff is empty, all content is in main even if commit hashes differ
+    if git diff --quiet "$main_branch"..."$branch" 2>/dev/null; then
+        echo "merged:content-identical"
+        return 0
+    fi
+
+    # Method 4: Cherry-pick detection (all commits have equivalents in main)
+    # Commits prefixed with '-' have equivalent patches in main
+    local unmerged=$(git cherry "$main_branch" "$branch" 2>/dev/null | grep -c '^+')
+    if [ "$unmerged" -eq 0 ]; then
+        echo "merged:cherry-picked"
+        return 0
+    fi
+
+    echo "not-merged"
+    return 1
+}
+
+# Find all merged branches with detection method
+for branch in $(git branch | grep -v "^\*" | sed 's/^[ ]*//'); do
+    if [ "$branch" != "$main_branch" ]; then
+        merge_status=$(check_if_merged "$branch" "$main_branch")
+        if [[ "$merge_status" == merged:* ]]; then
+            method=${merge_status#merged:}
+            echo "$branch|$method"
+        fi
+    fi
+done
 
 # Find branches with deleted remotes ("gone")
 git branch -vv | grep ': gone]' | awk '{print $1}'
@@ -123,8 +176,9 @@ git remote prune origin
    Current branch: feature/new-api
 
    === Merged Branches (safe to delete) ===
-   feature/bug-fix-123        Merged 2 weeks ago
-   feature/update-deps        Merged 1 month ago
+   feature/bug-fix-123        Merged (standard) - 2 weeks ago
+   feature/update-deps        Merged (merge-commit) - 1 month ago
+   feature/bug-fix-123        Merged (content-identical) - 3 days ago
 
    === Gone Branches (remote deleted) ===
    feature/old-feature        Remote: gone
@@ -139,6 +193,7 @@ git remote prune origin
 
    Recommendations:
    - Safe to delete: feature/bug-fix-123, feature/update-deps (merged)
+     * Note: feature/bug-fix-123 has different commits but identical content (rebased)
    - Safe to delete: feature/old-feature, hotfix/urgent-fix (remote gone)
    - Review needed: experiment/prototype (unmerged, stale)