fetchGit: allow fetching annotated tags

[balsoft]

Git doesn't like pointing a branch ref to an annotated tag, however the
other way around is fine. Use this to allow fetchGit to fetch annotated
tags, without specifically annotating them as such.

(hopefully) fixes #3701

Please review with care! I'm not sure this doesn't break some important assumption elsewhere.

Tested by poking around locally.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-01-20-nix-team-meeting-minutes-25/25432/1

[thufschmitt]

After digging a bit into that, it turns out that the thing that matters is that annotated tags aren't commits (they are their own thing), and Git doesn't allow writing something that's not a commit hash under /refs/heads (check here). Git being gentle-enough, it's nearly always fine to use a tag hash instead of a commit tag, so I think that fix is correct. Maybe for clarity we could use something else than tags though (since these refs don't have to be tags). I do like nixrefs 😛

And obviously: Can you add some tests for this?

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-06-12-nix-team-meeting-minutes-62/29315/1

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-10-02-nix-team-meeting-minutes-183/53815/1