Fix the test_get_branch to really highlight the issue

This test now fails, demonstrating that trying to run
twice the same codetree with "branch" without revno result in a
"git checkout master".
If the master does not exist, this fails (and checking out
master in such case is wrong anyway).

Reviewed-on: https://code.launchpad.net/~ballot/codetree/lp1829771/+merge/382971
Reviewed-by: Stuart Bishop <stuart.bishop@canonical.com>
Reviewed-by: Laurent Sesques <laurent.sesques@canonical.com>