No-op merge causes failure

[BhaaLseN]

Steps to reproduce:

1. Create a branch to be merged
2. Merge the branch by hand, but leave the remote branch
3. Trigger a merge

Since the Merge has no effect, the following amend (GitRepositoryManager.cs line 133) changes the manually merged commit, and thus fails the push afterwards (even after resetting and re-pulling, since the result is the same there).

[BhaaLseN]

Subsequent merges also fail, since the local working copy still has the broken history...(when attempting to do the initial pull with --ff-only, since it isnt a fast-forward at this point)