fetchgit: change allowedRequisites default from null to []

[ghost]

This is not ready to be merged yet!

Writing the one-line commit for this PR was the easy part. The hard part is making sure I don't break nixpkgs eval for a ton of people. I will undraftify this when I have made sure, and explain the process (which will be mostly
this) once I have done so.

This draft PR is mainly a way of putting this task on my nixpkgs to-do list. However comments are welcome, including comments that say "this is a dumb idea".

Description of changes

As originally suggested in #177326, and as further encouraged here, this commit changes the default allowedRequisites of fetchgit from null (any requisites allowed) to [] (no requisites allowed unless overridden).

Commit Message

fetchgit: change allowedRequisites default from null to []

If this commit broke eval for your package:

1. I apologize!

2. Please try adding `allowedRequisites=null` to your fetcher.

3. If (2) does not work, do not hesitate to revert this commit and
   ping @a-m-joseph to let me know I need to investigate.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.11 Release Notes (or backporting 22.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.