Revert angle bracket imports

This partially reverts commit 884c069c1740b8b131614e2f629df244a36f6ece.

Conflicts:
	Classes/GTCredential.h
	Classes/GTDiff.h
	Classes/GTDiffDelta.h
	Classes/GTEnumerator.h
	Classes/GTRepository.h
Rename Classes folder to ObjectiveGit for searches

This way, we’ll look for framework-style imports within the source tree
first, instead of jumping straight to the build products and
potentially causing duplicate definitions.
Fix some more janky imports