CI fails to fetch it:
```
error: Server does not allow request for unadvertised object 87d0d1fb0566a80229029d0d8d7c863138d70aae
warning: could not update mtime for file '/home/runner/.cache/nix/gitv3/0gjpwip102kwcvz961gsiva3lqmmr6266s5wzs8kq0ybm68gwpx9/refs/heads/master': No such file or directory
error:
… while checking flake output 'checks'
at /nix/store/fwrwzxjvvpx1l27h8j5f9gffzwn2vdik-source/flake.nix:54:7:
53| in {
54| checks = perSystemPackages (pkgs:
| ^
55| let
… while checking the derivation 'checks.x86_64-linux.full'
at /nix/store/fwrwzxjvvpx1l27h8j5f9gffzwn2vdik-source/flake.nix:83:11:
82| })).emacsWithDoom;
83| full = mkDoom {
| ^
84| full = true;
(stack trace truncated; use '--show-trace' to show the full trace)
error: Cannot find Git revision '87d0d1fb0566a80229029d0d8d7c863138d70aae' in ref 'refs/heads/master' of repository 'https://git.savannah.gnu.org/git/emms.git'! Please make sure that the rev exists on the ref you've specified or add allRefs = true; to fetchGit.
```
Try to follow the suggestions from the error message, as I do see the
commit in https://git.savannah.gnu.org/cgit/emms.git/.
I'm not sure why this is only failing now...
I knew this might cause problems at some point, but it came to a head
sooner than expected: it triggered
https://github.com/magit/magit/issues/5131 (magit is pinned but
magit-section was not, and those packages expect to be kept in sync).
The fix is messier than I'd like but at least fixes magit.
