Ref T4327. Consolidates and simplifies infrastructure:

• Moves Git discovery into DiscoveryEngine.
• Collapses a bunch of the Git and Mercurial code related to stream discovery.
• Removes all cach code from PullLocal daemon (it's no longer called).
• Adds basic unit tests for Git and Mercurial discovery.

Various cleanup:

• Makes GitStream and MercurialStream extend a common base.
• Improves performance of MercurialStream in some cases, by requiring fewer commits be output and parsed.
• Makes mirroring exceptions easier to debug.
• Fixes discovery of Mercurial repositories with multiple branch heads.
• Adds some missing pht().

I tested this fairly throughly because I think this phase is complete:

• Made new repositories in multiple VCSes and did full imports.
  • Particularly, I reimported Arcanist to make sure that TODO was resolved. I think it was related to the toposort stuff.
• Pushed commits to multiple VCSes.
• Pushed commits to a non-close branch, then pushed a merge commit. Observed commits import initially as non-close, then get flagged for close.
• Started full daemons and resolved various minor issues that showed up in the daemon log until everything ran cleanly.
• Basically spent about 30 minutes banging on this in every way I could think of to try to break it. I found and fixed some minor stuff, but it seems solid.