R1:d2b01aead048

Use one daemon to discover commits in all repositories, not one per repository

Summary:
See D2418. This merges the commit discovery daemon into the same single daemon, and applies all the same rules to it.

There are relatively few implementation changes, but a few things did change:

- I simplified/improved Mercurial importing, by finding full branch tip hashes with "--debug branches" and using "parents --template {node}" so we don't need to do separate "--debug id" calls.
- Added a new "--not" flag…