Extend nix-prefetch-git to support passing tree hashes as "rev"

[thomasjm]

Motivation for this change

I'm working on Julia packages in Nix. Julia's package manager organizes its dependencies a Manifest.toml file, which is almost perfect to use with Nix because it contains Git hashes we can pull out and pass to fetchgit. However, Julia uses Git tree hashes rather than commit hashes; see here.

It would be convenient if fetchgit could understand tree hashes when they're passed as the rev argument. It seems straightforward to support, as this PR shows.

I've tested this with my prototype Julia packaging tool and it works. If this could be accepted then I'd be happy to update the documentation and stuff. Thanks!

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions (Ubuntu 20.04)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• [N/A] Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[sheeldotme]

@jonringer it looks like you were the last person to commit to this file, I hope you don't mind being tagged. Do you by any chance know who might be the best person to provide feedback on this? Interested in getting this change merged.

[taku0]

Code-wise LGTM.

[taku0]

Please squash commits and rename to fetchgit: support passing tree hashes as "rev".

[thomasjm]

Done! Also rebased on master

[thomasjm]

Just checking in, is this ready to be merged?

[primeos]

FYI: This caused a regression that isn't the fault of this PR but it now becomes apparent that checkout_hash() is also relevant for some Git tags (which obviously fails now): #116307

(Just to link that issue here in case anyone else runs into that and finds this PR.)

[thomasjm]

Thanks @primeos. For completeness, there have been a couple other issues mentioning regressions:

#115145
#113926

I'm not sure what the best fix is. We could revert this if it's causing major problems.

FWIW, we could restore the old behavior by handling the "$object_type" == "tag" case like you mentioned and also catching any exceptions from git cat-file. But maybe that would just be papering over the problem.