Rewrite in N-API and update libgit2

[savetheclocktower]

git-utils is failing to build altogether on my work laptop (which runs macOS 26, despite my best efforts), so I figured this was a good time to bump libgit2.

This also adopts the needed rewrite of git-utils in N-API; this should probably fix pulsar#1468.

The tests pass for me locally, but I'll open this as a draft to start just to see what CI thinks of it on the other platforms.

[savetheclocktower]

The size of the diff should be explained.

We used to explicitly vendor libgit2 into deps/libgit2. When I did my N-API rewrite about 16 months ago, I converted that back into a submodule. If the submodule approach works, I'd much prefer it; if not, I can switch back to vendoring.

Either way, this PR would touch a lot of files; if the submodule approach works, then future libgit2 bumps would go much more smoothly.

[savetheclocktower]

OK, CI currently only tests in Linux. Good to know Linux passes, but I should add Windows to the matrix or else I can't be certain that this is a safe change to make! I'll revisit this tomorrow.

[savetheclocktower]

PR #5 has more context about why libgit2 was vendorized, and why it's likely fine to reintroduce the submodule:

Separately: @mauricioszabo indicated that his original reason for vendorizing libgit2 had something to do with yarn. It occurs to me that he was likely running into problems that were specific to how yarn handled dependencies that were git repositories instead of NPM packages. But now that we have a package-publishing strategy, there should be no need for vendorizing, and we can return to the submodule strategy.

[savetheclocktower]

Hey! Got a passing test suite on all platforms. I think some of those Windows bugs had been around for a while.

Taking this out of draft!