Add libgit2sharp functionality to Utility for revisioned mod installation

Needs libraries for linux/windows before merge (and referenced dylib rename in source of libgit2sharp?).

Git repo name must contain `oramod-`<modname>.
No better way to update right now than deletion and recloning.

Functionality tested with:
    `$` `mono OpenRA.Utility.exe --git-mod-install https://github.com/Phrohdoh/oramod-warfair`
    (though the actual mod is broken)

Also pinging @scskwirl; this is the thing I was working on that will make testing Warfair easier.