Update git-svnrevert to accept git and svn revisions

Interchangeable commit ids can now be used on this git-svnrevert, which
will figure out what kind of commit that is (if you use format rNNNN for SVN
commits) and make sure the right ids are used in the right places.

It's a little bit more robust and user-friendly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219290 91177308-0d34-0410-b5e6-96231b3b80d8