filter-branch: Fix error message for --prune-empty --commit-filter

Running filter-branch with --prune-empty and --commit-filter reports:

  "Cannot set --prune-empty and --filter-commit at the same time".

Change it to use the correct option name: --commit-filter

Signed-off-by: Jacob Helwig <jacob.helwig@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -207,7 +207,7 @@ t,)
 ,*)
 	;;
 *)
-	die "Cannot set --prune-empty and --filter-commit at the same time"
+	die "Cannot set --prune-empty and --commit-filter at the same time"
 esac
 
 case "$force" in