Add libgit2sharp functionality to Utility.

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 https://github.com/Phrohdoh/oramod-warfair`
	(though the actual mod is broken)