[BZ#685889] support diff3 format files

Submitted by Gregor B. Rosenauer

Description

sometimes it's already too late to use meld, and you end up with conflict markers in your file(s), like >>>> HEAD and <<< REV x.

For these cases, it would be nice if meld detected the diff3 format markers and automatically show me the common ancestor, local and remote version in a 3-way-merge-view instead of everything in one file.

I'd highly appreciate such a feature, as I think meld is currently the best merge tool available (the interface is way better than kdiff3's, much more like the Eclipse merge view which I find excellent) and often use it with git.