objective-git still relies on libgit2's deprecated API shims #652

The libgit2 1.9.4 bump in #641 restores compatibility through libgit2's
deprecation shims rather than migrating to the current API. Fifteen files in the
pinned objective-git import git2/deprecated.h to bring back pre-1.0 symbol
names:

GTConfiguration.m  GTRemote.m     GTBlob.m       GTFilterList.m  GTIndex.m
GTRepository+Status.m             GTFilter.m     GTBranch.m      GTCredential.m
GTRepository+RemoteOperations.m   GTDiff.m       GTDiffPatch.m   GTReference.m
GTOID.m            NSData+Git.m

That is a reasonable way to land a jump from 0.28.5, and it works: libgit2 keeps
those symbols unless built with DEPRECATE_HARD, which is OFF by default and
not set here.

Filing it so the debt is written down rather than rediscovered at the next bump.
Two things make it worth tracking:

• The shims are opt-out today, but they are deprecated upstream and can be
  removed. A future libgit2 that drops them, or a build that turns
  DEPRECATE_HARD on, turns this into a compile failure across fifteen files at
  once.
• The imports mark exactly which call sites still use the old API, so they are a
  ready-made checklist for migrating.

No action needed while #641 is in flight. Raised during its review and promised
there, so recording it here.