git, gitweb: Fix git-instaweb

[kirelagin]

Motivation for this change

Git’s build system does two things with the gitwebdir variable:

1. Puts gitweb there
2. Embeds this path into git-instaweb

Before this change git-instaweb wouldn’t work as it was looking for gitweb in git’s share directory, but it was moved away into a separate output by the nix expression.

This PR sets the gitwebdir variable in the Makefile and points it to the gitweb output. As a result, a path to this output is now embedded into git-instaweb, which will affect the closure size, but, AFAIU, only if perlSupport is enabled, so this is probably not a problem? I also had to move the fixups from the gitweb expression to git itself to make sure that gitweb.cgi that is referenced from git-instaweb is functional.

Fixes #53491.

@gnidorah @peti @the-kenny @wmertens

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

• I always interrupted the build of git as its tests take too long, but I am relatively confident that it works as its tests were running successfully for quite a while.
• git instaweb now works
• I have not tried to build gitweb (as git never finished building), but I don’t think this change could break something in it.

[kirelagin]

Oh, ok, never mind. Skipped tests and it doesn’t work:

patching script interpreter paths in /nix/store/...-git-2.19.2-gitweb
cycle detected in the references of '/nix/store/...-git-2.19.2-gitweb' from '/nix/store/...-git-2.19.2'
error: build of '/nix/store/...-git-2.19.2.drv' failed

Not sure what to do now. Probably unsplit git and git.gitweb? 🤔 @gnidorah

[kirelagin]

For reference, I partially reverted #38918.

[ghost]

@kirelagin sorry, i don't use gitweb atm. Feel free to do whatever you find an appropriate.

[wmertens]

Since you're going from not working to working, I think you can merge if it works and if closure size is not impacted overly much.

[aanderse]

@kirelagin from your perspective what is the status on this?

[kirelagin]

I don’t see any reasons not to merge it.

[aanderse]

@wmertens @peti @the-kenny final approval before merging please?

[wmertens]

Thanks, looking forward to trying it out!