show-ref --verify: accept pseudorefs

show-ref --verify: accept pseudorefs

"git show-ref --verify" is useful for scripts that want to look up a
fully qualified refname without falling back to the DWIM rules used by
"git rev-parse" rules when the ref does not exist. Currently it will
only accept "HEAD" or a refname beginning with "refs/". Running

    git show-ref --verify CHERRY_PICK_HEAD

will always result in

    fatal: 'CHERRY_PICK_HEAD' - not a valid ref

even when CHERRY_PICK_HEAD exists. By calling refname_is_safe() instead
of comparing the refname to "HEAD" we can accept all one-level refs that
contain only uppercase ascii letters and underscores.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>