Test to confirm JENKINS-20286 - JGit incorrectly detects changes

The JGit implementation of getHeadRev(Url, Branch) returns null if a
wildcard is included in the branch name, while the CliGit
implementation of the same method returns a SHA1 for the first branch
which matches that wildcard.

This is only a test which confirms the behavior is different between
CliGit and JGit.  The fix will need more time.