Isn't that opendiff? https://developer.apple.com/library/mac/documentation/Darwin...

Could be this http://www.kaleidoscopeapp.com/

This is kinda a somewhat wild guess, but it looks to me like SmartGit from syntevo with some custom settings.

http://www.syntevo.com/smartgit/

Watched video on my phone so, not sure, but I think Bitbucket has a side-by-side diff like that one. Edit: typo