Improve error message for git-filter-branch

Tell the user that a backup (original) already exists, and how to solve
this problem (with -f option)
Signed-off-by: NJohn Tapsell <johnflux@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -232,7 +232,9 @@ while read sha1 type name
 do
 	case "$force,$name" in
 	,$orig_namespace*)
-		die "Namespace $orig_namespace not empty"
+		die "Cannot create a new backup.
+A previous backup already exists in $orig_namespace
+Force overwriting the backup with -f"
 	;;
 	t,$orig_namespace*)
 		git update-ref -d "$name" $sha1