[hannesa2]

Summary

Bumps the vendored libgit2 submodule to the v2.0.0-rc.1 release candidate
tag and adjusts ObjectiveGit for the resulting libgit2 2.0 API changes, so
that repositories migrated with git refs migrate --ref-format=reftable can
be opened again.

v1.9.x doesn't recognize extensions.refstorage=reftable at all, so opening
such a repository failed with unsupported extension name extensions.refstorage. libgit2 only gained a native reftable refdb backend
after 1.9, first shipping in v2.0.0-rc.1, so the submodule is pinned there.

Note: v2.0.0-rc.1 is a release candidate, not a final libgit2 2.0
release. This PR exists to validate reftable support; it should be revisited
once libgit2 2.0.0 (or a later rc) is tagged.

Changes

• External/libgit2: bumped to the v2.0.0-rc.1 tag.
• ObjectiveGit/GTObject.h: GTObjectTypeOffsetDelta/GTObjectTypeRefDelta
  are now hardcoded to 6/7. libgit2 2.0 removed these from the public
  git_object_t enum (they're pack-internal delta types, never real ODB
  objects); the numeric values are stable per the git pack-format spec.
• ObjectiveGit/GTOID.m: OIDByHashingData:type:error: now uses
  git_object_id_from_buffer instead of the deprecated git_odb_hash.
• ObjectiveGit/GTRepository.h: GTRepositoryInitWithoutDotGit is now a
  hardcoded no-op (0) and marked deprecated. In libgit2 1.9.7 this was a
  real bit (GIT_REPOSITORY_INIT_NO_DOTGIT_DIR, 1u << 2) that stopped
  .git being appended to the init path; libgit2 2.0 dropped the flag
  entirely, and the same effect is now implied whenever workdir_path is set
  on the init options instead. Nothing in GitX or this repo uses this flag.
• script/update_libgit2: added -DUSE_GSSAPI:BOOL=OFF to the CMake
  invocation. Without it, CMake on macOS auto-detects GSS.framework /
  system Kerberos and enables GSSAPI/Negotiate HTTP auth support in
  v2.0.0-rc.1, which requires linking GSS symbols that the Xcode project
  doesn't link, causing ld: Undefined symbols ... _gss_* at link time.
  Disabling GSSAPI avoids that dependency; GitX doesn't use Negotiate auth.

Known limitation

v2.0.0-rc.1 vendors libgit2's low-level deps/reftable library, but
upstream has not yet wired it into an actual refdb backend (no
refdb_reftable.c, extensions.refstorage not yet in the builtin extensions
allowlist). Opening a reftable-format repository therefore still fails today;
this is tracked upstream as libgit2/libgit2#5352 and will need a newer
libgit2 release once that lands.

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[goneng]

Thanks for this, @hannesa2 .
I built it with the GitX PR on top and the full GitX suite passes.
The git_object_id_from_buffer switch in GTOID.m looks right (OID type defaults to SHA-1), and
having no git2/deprecated.h imports left is a nice outcome for libgit2#652.

A few "cosmetic" suggestions:

• GTRepositoryInitWithoutDotGit = 0:
  The new comment says the flag "was already a deprecated no-op".
  In libgit2 1.9.7 it was a real bit, (1u << 2), that stopped .git being appended.
  In 2.0 it is #define ... 0 in deprecated.h, and the behavior is implied when workdir_path is set.
  Could you reword the comment?
  (A caller that used the flag without workdir_path now silently gets different behavior, so
  deprecating the enum member (or removing it) may be worth considering.
  Nothing in GitX or in this repo uses it.)
• Commit order:
  The first commit uses git_object_id_from_buffer, which does not exist in libgit2 1.9.7,
  and the libgit2 bump comes second, so the first commit alone does not build.
  Swapping the order or squashing them keeps git bisect usable.
• -DUSE_GSSAPI:BOOL=OFF in script/update_libgit2:
  this is needed, since rc.1 otherwise finds GSS.framework and enables Negotiate auth.
  I suggest saying so in the PR description, because it is not obvious from the diff.
• Minor and optional:
  git_dataWithOid: and git_getOid:error: in NSData+Git.m use sizeof(git_oid), which
  now includes a type byte.
  I found no callers, so nothing breaks today.

Apart from these small issues, I think it is safe to merge.

[hannesa2]

I applied your suggestions @goneng

[goneng]

LGTM - thanks!