Recent check_ref_format() returns -3 as well as -1 (general
error) and -2 (less than two levels). The caller was explicitly
checking for -1, to allow "HEAD" but still needed to disallow
bogus refs.
This introduces symbolic constants for the return values from
check_ref_format() to make them read better and more
meaningful. Normal ref creation codepath can still treat
non-zero return values as errors.