Handle reversing merge commits better

Raised from: https://answers.atlassian.com/questions/56084/reverse-merge-in-source-tree?page=1#comment-138684

Currently reversing a merge commit will cause problems, both in git and hg.

Git has a 'revert' option '-m' which can be used to reverse merges correctly, see: http://git-scm.com/2010/03/02/undoing-merges.html

We should support that when a user selects a merge to reverse. We should also investigate whether there is an equivalent option for hg.