With a Windows-style `$PWD` (e.g. `D:/Work`, as Git Bash inherits when launched
by a native Windows program), `${path_%/*}` stops changing at `D:`, so the walk
never ends and sourcing `nvm.sh` hangs. Stop when going up doesn't change the
path, which also covers the existing `.` case.
Fixes#3967.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RBGzjdkmgq6LSA7SFtjRsY