logmsg_reencode: return const buffer

The return value from logmsg_reencode may be either a newly
allocated buffer or a pointer to the existing commit->buffer.
We would not want the caller to accidentally free() or
modify the latter, so let's mark it as const.  We can cast
away the constness in logmsg_free, but only once we have
determined that it is a free-able buffer.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/reset.c

@@ -93,7 +93,7 @@ static int reset_index(const unsigned char *sha1, int reset_type, int quiet)
 static void print_new_head_line(struct commit *commit)
 {
 	const char *hex, *body;
-	char *msg;
+	const char *msg;
 
 	hex = find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV);
 	printf(_("HEAD is now at %s"), hex);