R1:99231ebba424

Allow Git repositories to track only some branches

Summary:
Some installs use Git as the backbone of a CI framework or use a Git remote to
share patches. The tracker scripts currently recognize associated revisions as
"Committed" when they appear in any branch, even if that branch is
"alincoln-personal-development_test_hack" or whatever.

To address the broadest need here, allow Git repositories to be configured to
track only certain branches instead of all branches.

This doesn't allow you to import a…