Teach log family --encoding

Updated commit objects record the encoding used in their
encoding header.  This updates the log family to reencode it
into the encoding specified in i18n.commitencoding (or the
default, which is "utf-8") upon output.

To force a specific encoding that is different, log family takes
command line flag --encoding=<encoding>; giving --encoding=none
entirely disables the reencoding and lets you view log messges
in their original encoding.

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

revision.h

@@ -72,6 +72,7 @@ struct rev_info {
 	const char	*ref_message_id;
 	const char	*add_signoff;
 	const char	*extra_headers;
+	const char	*log_reencode;
 
 	/* Filter by commit log message */
 	struct grep_opt	*grep_filter;