Merger: support for finding branches in remote

The new merger.hasBranch() method let us find out whether the remote
repository has the given branch. That is probably of no use for Zuul
merger itself but will let me reproduce the devstack-gate
git_has_branch() bash function.

Change-Id: Ibca58172a7b41b1c6b63cdd4449a647020bad0c9
1 file changed