I put similar code in Emacs's vc-hg to get revision information straight form Mercurial's on-disk data structures instead of firing up an hg subprocess.
You mean actually reading dirstate[0] or just the branch/bookmark files?
We also do the latter, but dirstate format isn't easily readable just with shell builtins (lots of fixed-length fields with NUL-byte padding, also we don't even have a `stat` builtin and the external program isn't a thing on macOS AFAIK), so we still fire up `hg status` for that - but only after we decide that there is a hg repo.
