GitInputScheme on detached HEAD sets ref to rev

[edolstra]

Describe the bug

E.g. on nixpkgs:

# git checkout a28adc36c20fd2fbaeb06ec9bbd79b6bf7443979
# nix flake metadata
Resolved URL:  git+file:///home/eelco/Dev/nixpkgs
Locked URL:    git+file:///home/eelco/Dev/nixpkgs?ref=a28adc36c20fd2fbaeb06ec9bbd79b6bf7443979&rev=a28adc36c20fd2fbaeb06ec9bbd79b6bf7443979
...

Expected behavior

The ref field should be left unset.