Fixes #882

Added and implement kGCCheckoutOption_RemoveUntrackedFiles to fix issue where untracked files are left behind after forceCheckoutHEAD is called during a rebase.

I was able to reproduce the issue in the -[GCEmptyLiveRepositoryTestCase testMultipleCommitsRebaseWithConflict] test I implemented in #989 (I would've based this branch on the branch I used for PR 989 but I can't open a PR on another PR from my fork).

Then I dug deeper and deeper until I found that -[GCRepository forceCheckoutHEAD:error:] didn't remove untracked files, but that this was required for rebases. I still don't fully understand what caused this regression, but this does seem to fix it (and all the other tests pass).

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