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GH-41735: [CI][Archery] Update archery to be compatible with pygit2 1.15 API change (#41739)

### Rationale for this change

pygit2 updated how they expose some of the `GIT_OBJ` variables to use `GIT_OBJECT` prefix here: https://github.com/libgit2/pygit2/commit/8b3861b9092d1e3517b11f6ab06434ea866dd051

### What changes are included in this PR?

Update code to make it compatible with both possible APIs.

### Are these changes tested?

Via archery

### Are there any user-facing changes?

No
* GitHub Issue: #41735

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>