R1:e48830eecb95

Don't rediscover an entire branch when a new commit appears at HEAD

Summary:
The autoclose logic is currently doing a little too much work. We want to parse each commit at most twice:

# When it first appears in the repository.
# When it first appears on an autoclose branch.

These two events might not be distinct (i.e., it might first appear on an autoclose branch).

Currently, to discover commits initially appearing on autoclose branches, we check each branch, determine if it's an autoclose branch…