path: move `validate_headref()` to its only user

path: move `validate_headref()` to its only user

While `validate_headref()` is only called from `is_git_directory()` in
"setup.c", it is currently implemented in "path.c". Move it over such
that it becomes clear that it is only really used during setup in order
to discover repositories.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>