find_git_common_dir

Resolve a validated identity shared by one repository's Git worktrees.

A normal checkout uses its in-tree .git directory. A linked worktree is accepted only when its administration directory lives directly under the common repository's worktrees directory and its gitdir file points back to the current worktree's .git file, and both the shared common directory and the worktree's own administration directory carry valid repository metadata (_is_valid_git_common_dir plus the worktree's own HEAD). These checks prevent a forged pointer or directory link from borrowing another repository's identity.