Fix migrate sync failing in git worktree environments (#970)

Two issues caused `bd migrate sync` to fail when run from a git worktree:

1. Used GetGitDir() instead of GetGitCommonDir() for worktree path
   - GetGitDir() returns the worktree-specific path (.bare/worktrees/main)
   - GetGitCommonDir() returns the shared git dir (.bare) where new
     worktrees can actually be created

2. Used strings.Index instead of LastIndex in GetRepoRoot()
   - When user paths contain "worktrees" (e.g., ~/Development/worktrees/),
     Index finds the first occurrence and incorrectly strips the path
   - LastIndex finds git's internal /worktrees/ directory

Added GetGitCommonDir() to internal/git/gitdir.go for reuse.

Fixes GH#639 (remaining unfixed callsite in migrate_sync.go)

internal/syncbranch/worktree.go

@@ -987,8 +987,9 @@ func GetRepoRoot(ctx context.Context) (string, error) {
 			line := strings.TrimSpace(string(content))
 			if strings.HasPrefix(line, "gitdir: ") {
 				gitDir := strings.TrimPrefix(line, "gitdir: ")
-				// Remove /worktrees/* part
-				if idx := strings.Index(gitDir, "/worktrees/"); idx > 0 {
+				// Remove /worktrees/* part - use LastIndex to handle user paths containing "worktrees"
+				// e.g., /Users/foo/worktrees/project/.bare/worktrees/main should strip at .bare/worktrees/
+				if idx := strings.LastIndex(gitDir, "/worktrees/"); idx > 0 {
 					gitDir = gitDir[:idx]
 				}
 				repoRoot = filepath.Dir(gitDir)