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>

revision.h

@@ -56,6 +56,7 @@ struct rev_info {
 	enum cmit_fmt	commit_format;
 	const char	*header_prefix;
 	const char	*header;
+	const char	*use_precomputed_header;
 
 	/* special limits */
 	int max_count;