Add Objective Git submodule and subproject
Start using GTRepository
Reorder project files
More Objective Git/libgit2 usage

checkout, parseSymbolicReference, unstaged files
Checkout fix
ObjGit replacement for parseReference

parseReference was only ever used to see if HEAD exists, so that's what the replacement does.
Update Objective Git
Merge branch 'refs/heads/objgit'

Conflicts:
	.gitmodules
	Xit.xcodeproj/project.pbxproj
Use stageFile: instead of addFile:. They were the same!
Just have one commit method