MergeCommand should create missing branches

MergeCommand should create missing branches

If HEAD exists but points to an not-existing branch the merge
command should silently create the missing branch and check
it out. This happens if you pull into freshly initalized repo.
HEAD points to refs/heads/master but refs/heads/master doesn't
exist. If you know merge a commit X into HEAD then the branch
master should be created (pointing to X) the working tree should
be updated to reflect X. That is achieved by checkout with one
tree only (HEAD is missing).

A test for this functionality will come the the next proposal
in PullCommandTest.

Change-Id: Id4a0d56d944e0acebd4b3157428bb50bd3fdd872
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>