log/whatchanged/show - log formatting cleanup.

This moves the decision to print the log message, while diff
options are in effect, to log-tree.  It gives behaviour closer
to the traditional one.

Signed-off-by: Junio C Hamano <junkio@cox.net>

log-tree.c

@@ -54,6 +54,9 @@ static const char *generate_header(struct rev_info *opt,
 	int abbrev = opt->diffopt.abbrev;
 	const char *msg = commit->buffer;
 
+	if (opt->use_precomputed_header)
+		return opt->use_precomputed_header;
+
 	if (!opt->verbose_header)
 		return sha1_to_hex(commit_sha1);