If we specify a branch, always use it

There's a check to make sure that we only run
mirroring against interesting branches by filtering
out any that aren't HEAD or don't start with
remotes/origin, but if we specify a branch on
the command line, we want that branch no matter what.
This is primarily so we can run mirroring tests
on proposed changes to openstack/requirements.

Change-Id: I19cd2b032b6219aed972abb95a180e3d402d529e

pypi_mirror/cmd/run_mirror.py

@@ -233,7 +233,7 @@ class Mirror(object):
             for branch in branches:
                 branch = branch.strip()
                 if (not branch.startswith("remotes/origin")
-                        or "origin/HEAD" in branch):
+                        or "origin/HEAD" in branch) and len(branches) > 1:
                     continue
                 print("Fetching pip requires for %s:%s" % (project, branch))
                 if not self.args.no_update: