sha1_name: Suggest commit:./file for path in subdir

Currently, the "Did you mean..." message suggests "commit:fullpath"
only. Extend this to show the more convenient "commit:./file" form also.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1506-rev-parse-diagnosis.sh

@@ -9,7 +9,7 @@ exec </dev/null
 test_did_you_mean ()
 {
 	printf "fatal: Path '$2$3' $4, but not ${5:-'$3'}.\n" >expected &&
-	printf "Did you mean '$1:$2$3'?\n" >>expected &&
+	printf "Did you mean '$1:$2$3'${2:+ aka '$1:./$3'}?\n" >>expected &&
 	test_cmp expected error
 }