R1:618da2265d51

Remove all the multi-pass autoclose-branch separate-cache / seenOnBranches junk

Summary:
Ref T4327. Simplify the git discovery process so I can move it to the DiscoveryEngine, so I can make change parsing testable.

In particular:

- As an optimization, we process closeable branches ("master") first, then process uncloseable branches ("epriestley-devel"). This means that in the common case we can insert a commit as closeable immediately when it is discovered, the first pass through the pipeline will get…