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)
c988c76b08