Transition to using libgit2 as much as possible

[Uncommon]

The first attempt showed a 2x slowdown in load time, but a more recent try got almost 6x faster instead.

[Uncommon]

Partially done in 474653617a4465c6f5c9c436b95f37a9847dac8a

Remaining in XTRepository.m:

• [x] contentsOfFile:atCommit:
• [x] getCommitsWithArgs:...

XTRepository+Parsing.m:

• [x] readRefsWithLocalBlock:...
• [x] readStagedFilesWithBlock:
• [x] readStashesWithBlock:
• [x] fileNamesForRef:
• [x] parseCommit:...
• [ ] stageFile:
• [ ] unstageFile:
• [ ] commitWithMessage:amend:...

XTRepository+Commands.m: (Many of these will need to be refactored when they are UI-accessible)

• [ ] saveStash:
• [ ] createBranch:
• [x] push: (used only in tests)
• [x] createTag:
• [x] deleteTag:
• [x] addRemote:withUrl:
• [x] deleteRemote:error:
• [x] diffForStagedFile:
• [x] diffForUnstagedFile:
• [x] diffForCommit:
• [ ] stagePatch:
• [ ] unstagePatch:
• [ ] discardPatch:
• [x] renameBranch:
• [ ] renameRemote:
• [x] dropStash:error:
• [ ] merge:error:
• [x] popStash:error:
• [x] applyStash:error:

[Uncommon]

I tried to do saveStash: but the tests all failed

- (BOOL)saveStash:(NSString*)name includeUntracked:(BOOL)untracked
{
  const GTRepositoryStashFlag flags = untracked ?
      GTRepositoryStashFlagIncludeUntracked :
      GTRepositoryStashFlagDefault;
  NSError *error = nil;
  GTCommit *result =
      [self.gtRepo stashChangesWithMessage:nil flags:flags error:&error];

  return result != nil;
}

[Uncommon]

I'm actually thinking about phasing out Objective Git and using libgit2 directly. It's one less dependency, and easier to manage which version of libgit2 is used.

[Uncommon]

Note that there is an implementation of Blame using libgit2, but it can be a lot slower in large repositories so the CLI version will be used until that is fixed.