Basic file merging

[tiennou]

This adds a wrapper for the simple git_merge_file & git_merge_file_from_index.

This is somewhat a leftover from #664, because the implementation was already available in libgit2 as the latter, and thus misses nicer things (like filenames in conflict markers).

I'd like to get that in ASAP, so I can update libgit2 and make a release from that, and that API is in my way 😉.