Allow running on detached head

[kevmeyer]

This allows running on a detached head. This enables running this hook e.g. in "speculative-merge" CI job cases.
Previously:

Untracked Django migrations checker...........................................................Failed
- hook id: check-untracked-migrations
- exit code: 1

fatal: ref HEAD is not a symbolic ref

[kevmeyer]

@ecugol what do you think?

[blag]

LGTM