builtin-merge: show user-friendly error messages for fast-forward too.

fadd069d03 (merge-recursive: give less scary messages when merge did not
start, Sep 7 2009) introduced some friendlier error message for merge
failure, but the messages were shown only for non-fast forward merges.
This patch uses the same for fast-forward.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-merge.c

@@ -650,6 +650,7 @@ static int checkout_fast_forward(unsigned char *head, unsigned char *remote)
 	opts.verbose_update = 1;
 	opts.merge = 1;
 	opts.fn = twoway_merge;
+	opts.msgs = get_porcelain_error_msgs();
 
 	trees[nr_trees] = parse_tree_indirect(head);
 	if (!trees[nr_trees++])