fixing output of non-fast-forward output of post-receive-email

post-receive-email has one place where the variable fast_forward is not
spelled correctly.  At the same place the logic was reversed.  The
combination of both bugs made the script work correctly for fast-forward
commits but not for non-fast-forward ones.  This change fixes this to
be correct in both cases.

Signed-off-by: Robert Schiele <rschiele@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

contrib/hooks/post-receive-email

@@ -323,7 +323,7 @@ generate_update_branch_email()
 		echo "       via  $rev ($revtype)"
 	done
 
-	if [ -z "$fastforward" ]; then
+	if [ "$fast_forward" ]; then
 		echo "      from  $oldrev ($oldrev_type)"
 	else
 		#  1. Existing revisions were removed.  In this case newrev is a