libgit2, GitRepo: Write (thin) packfiles (backport #11330)

[mergify]

Motivation

This improves performance on systems with weak I/O in ~/.cache, especially in terms of operations per second, or where system calls are slower. (macOS, VMs?)

May address

• Flake copying performance regressed on macOS in 2.19 #11098
• Fetching paths from cache.nixos.org gets slow in certain Docker environments #11258

Context

• Relevant since Use the Git cache for tarball flakes #10038
• Upstream PRs
  • Make packbuilder interruptible using progress callback libgit2/libgit2#6874
  • Add git_mempack_write_thin_pack libgit2/libgit2#6875

TODO

• measurements:
  • faster on emilazy's machine
  • similar on 2019 high end linux laptop with ZFS on NVMe
  • ...?
• review memory management
• packfile threads setting?
• make it interruptible

Priorities and Process

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---

This is an automatic backport of pull request #11330 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 5dd6c4f has failed:

On branch mergify/bp/2.24-maintenance/pr-11330
Your branch is up to date with 'origin/2.24-maintenance'.

You are currently cherry-picking commit 5dd6c4f06.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   packaging/patches/libgit2-mempack-thin-packfile.patch
	modified:   src/libfetchers/git-utils.hh

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   packaging/dependencies.nix
	both modified:   src/libfetchers/git-utils.cc

Cherry-pick of c1fe354 has failed:

On branch mergify/bp/2.24-maintenance/pr-11330
Your branch is ahead of 'origin/2.24-maintenance' by 5 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit c1fe3546e.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   packaging/patches/libgit2-packbuilder-callback-interruptible.patch

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   packaging/dependencies.nix

Cherry-pick of 976f539 has failed:

On branch mergify/bp/2.24-maintenance/pr-11330
Your branch is ahead of 'origin/2.24-maintenance' by 6 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 976f539f7.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libfetchers/git-utils.cc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[roberth]

Could be done if there's interest.