R1:6daa2b6c2efa

Fix a commit hook issue with the initial commit to Mercurial repositories

Summary:
Fixes T4257. The `hg heads` command exits with an error code and no output in an empty repository.

Just ignore the error code: we don't have a great way to distinguish between errors, and we ran another `hg` command moments before, so we have at least some confidence it isn't a PATH sort of thing.

Test Plan: Created a new Mercurial repository and pushed to hit the error in T4257. Applied this fix and got a clean push with…