Correct error message in revert/cherry-pick

We now write to MERGE_MSG, not .msg.  I missed this earlier
when I changed the target we write to.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/builtin-revert.c b/builtin-revert.c
index 55d4fa1..ea2f15b 100644
--- a/builtin-revert.c
+++ b/builtin-revert.c
@@ -133,7 +133,7 @@
 {
 	int len = strlen(string);
 	if (write_in_full(msg_fd, string, len) < 0)
-		die ("Could not write to .msg");
+		die ("Could not write to MERGE_MSG");
 }
 
 static void add_message_to_msg(const char *message)