git-filter-branch: be more direct in an error message

git-filter-branch requires the specification of a branch by one way or
another. If no branch appears to have been specified, we know the user
got the usage wrong but we don't know what they were trying to do ---
e.g. maybe they specified the ref to rewrite but in the wrong place.

In this case, just state that the branch specification is missing.

Signed-off-by: Jean-Noel Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -239,7 +239,7 @@ git rev-parse --no-flags --revs-only --symbolic-full-name \
 sed -e '/^^/d' "$tempdir"/raw-heads >"$tempdir"/heads
 
 test -s "$tempdir"/heads ||
-	die "Which ref do you want to rewrite?"
+	die "You must specify a ref to rewrite."
 
 GIT_INDEX_FILE="$(pwd)/../index"
 export GIT_INDEX_FILE