Added test for branch names using "reserved" keywords

Test for getHeadRev with branch names which SHOULD BE reserved by Git,
but ARE NOT. E.g. it is possible to create the following LOCAL (!)
branches:
- "origin/master" results in refs/heads/origin/master
- "remotes/origin/master" results in refs/heads/remotes/origin/master
- "refs/heads/master" results in refs/heads/refs/heads/master
- "refs/remotes/origin/master" results in
  refs/heads/refs/remotes/origin/master
Fix for previous branch issue + general reserved branch names

Tests work for Cli client, but not yet for JGit client.