git-p4: work with a detached head

When submitting, git-p4 finds the current branch in
order to know if it is allowed to submit (configuration
"git-p4.allowSubmit").

On a detached head, detecting the branch would fail, and
git-p4 would report a cryptic error.

This change teaches git-p4 to recognise a detached head and
submit successfully.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Jeff King <peff@peff.net>

t/t9800-git-p4-basic.sh

@@ -241,7 +241,7 @@ test_expect_success 'unresolvable host in P4PORT should display error' '
 	)
 '
 
-test_expect_failure 'submit from detached head' '
+test_expect_success 'submit from detached head' '
 	test_when_finished cleanup_git &&
 	git p4 clone --dest="$git" //depot &&
 	(