Add git_diff_buffer_to_buffer

[brianmario]

One use-case here is that I take a diff, transcode each side into UTF-8 for display purposes but then want to perform a diff on the transcoded text to finally be able to get a diff of the actual characters that changed other than the encoding.

Having a generic way to use libgit2's diffing engine seems like a win anyway :)

[carlosmn]

I guess this is one way to sidestep the textconv filtering design ;)

[arrbee]

I'm working on it...

[arrbee]

See PR #2146 - forgot to include a "fixes" message in there