Switch to libgit2

[MailYouLater]

There have been issues with ungit which stem from the fact that ungit calls git on a command line, and interprets its output. Most notably, there were issues if git was set to use a language other than English. See #959 & #963.

I suggest that you switch to libgit2 to accomplish git tasks, instead of calling the git command for these functions, this would avoid any similar issues in the future.

[jung-kim]

so with #1208 multi lang shouldn't be an issue.

There are other arguments for libgit2 and we've had discussions before #260 and it maybe worth to revisit again but yes it will be a big overhaul.

[wmertens]

I think this issue is just a duplicate of #260 and can therefore be closed.