test_uploads_with_nondefault_rebase: fix git screen scraping
test_uploads_with_nondefault_rebase: fix git screen scraping

Newer versions of git quote branch names in their output; at least git
2.16.3 does. This causes all test_uploads_with_nondefault_rebase tests to
fail like this:

testtools.matchers._impl.MismatchError:
 'Branch test_branch set up to track remote branch maint from origin.'
 not in
  u"Switched to a new branch 'test_branch'\n\
 Branch 'test_branch' set up to track remote branch 'maint' from 'origin'."

Add a testtools.matchers.MatchesRegex() to support both styles.

Change-Id: I9f1417c53de2f7d638e845f553df3bd426a4c750

openstack-infra/git-review