Bump objective-git to stop building the libgit2 test suite - #708

Every GitX build runs libgit2's cmake build, which compiled a Clar test
suite that is never run, because the flag meant to switch it off stopped
matching libgit2's own option name at the 1.x rename.

• Move the objective-git pointer onto Stop building the libgit2 test suite on every build objective-git#108, which
  passes -DBUILD_TESTS:BOOL=OFF in place of the dead
  -DBUILD_CLAR:BOOL=OFF and cuts 471 of the 667 objects the build
  compiles.
• Take that fix alone rather than the current objective-git master, which
  has since moved on to unrelated work in Fix two defects left by the libgit2 1.x API migration objective-git#107.

What it saves

libgit2 is rebuilt from scratch several times in every CI job, because
the script always does rm -rf build and runs both as its own step and
again from the ObjectiveGit target's Run Script phase. The arm64 job
builds it three times, in pre build, Run unit tests and
Build project; the x86_64 job twice, lacking the unit test step. So the
saving lands once per build, several times per job.

Measured against run 35601332529, the pull request run 14 minutes before
this one on the same workflow, with two runs of this branch shown because
the spread between them is worth seeing:

Call it 4 to 6 minutes off the x86_64 job and around 3 minutes off the
arm64 one. The two runs after the change built byte-identical trees, so
the spread between them is runner variance, and the x86_64 figure in
particular should be read as a range rather than a number. What is not
noisy is the object count, which is exactly 196 per build against 667
before, every time.

Job totals move further than the steps above, from 14m07s to 6m53s and
16m41s to 8m24s, but some of that is unrelated: this pull request skips
notarization, having no signing secrets from a fork, and skips
Run tests, since a submodule bump cannot change the screenshots.

Test plan

• CI built GitX against the new pointer on both architectures, and the
  logs show 196 objects compiled per libgit2 build with none of them
  tests, against 667 with 470 tests before.
• The pointer moved from the pull request head to the merge commit after
  Stop building the libgit2 test suite on every build objective-git#108 landed. The two are byte-identical trees, and CI
  is green on both.
• Locally, building libgit2 before and after produced an
  External/libgit2.a of the same size to the byte, 6,433,360, exporting
  an identical set of 2,097 global text symbols.
• objective-git's own CI was green on Stage Diff view: Selection jumps around when cursor is over Copy/Stage buttons #108 on both macos-15-intel and
  macos-26, through xcodebuild archive.