[BZ#699037] allow conflict resolution to be reverted

Submitted by Adam Dingle

Description

All too often, after I've resolved conflicts in a file (but before committing) I realize I've made an error and I want to go back and merge the changes in that file again. Today, once I've resolved a conflict in Meld there's no way I can go back to the previous unresolved state. In git, the right command to get there is 'git checkout -m':

http://gitster.livejournal.com/43665.html

I think in Meld the Revert command should probably always run 'git checkout -m'. I believe this command will work correctly in the simple case of reverting everyday changes, and it can also restore the state before a merge resolve.