Get branches directly from Git rather than database

We really don't use the Branch entity in Gerrit.  The Branch.Id
secondary key is completely unused, so I'm dropping it from the
schema to reduce the code complexity.  Unfortunately the record
in the database is used to determine if a change can e uploaded
to the branch, or not, so we are forced to keep it around as a
boolean flag for now.

The admin panel for a project now lists the branches directly
from the Git repository, as the database table will hopefully
disappear in the near-ish future, especially as we start to
move to a Git backed datastore.

Signed-off-by: Shawn O. Pearce <sop@google.com>
11 files changed