fix(git): support relative worktree repositories - #559

This has since landed by another route, so I think it can be closed.

The diagnosis here was right: relative-worktree repos need a libgit2 that
recognises the extension. relativeworktrees entered libgit2's
builtin_extensions in 0516d66f, first released in 1.9.4, and master picked
that up in #641 (through gitx/objective-git#101, since #98 was closed). I
built a probe against the currently pinned libgit2 and pointed it at a repo
with extensions.relativeWorktrees=true: both the main checkout and the
worktree open with rc=0 and resolve workdir and commondir correctly.

The rest of the diff is covered too. The .asize -> .size fix landed in #582,
GitRepoFinder.m is not in the Xcode target, and the Makefile has since grown
targets for every CI step.

One line does survive: git_buf_free -> git_buf_dispose in
PBRepositoryFinder.m:52. That is part of the wider deprecated-shim cleanup
rather than anything worth keeping this PR open for.

Thanks for chasing this down.

One line does survive: git_buf_free -> git_buf_dispose in
PBRepositoryFinder.m:52. That is part of the wider deprecated-shim cleanup
rather than anything worth keeping this PR open for.

I used it here #761
After that merge, we can close this one