pretty=format: respect date format options

When running a command like:

  git log --pretty=format:%ad --date=short

the date option was ignored. This patch causes it to use whatever
format was specified by --date (or by --relative-date, etc), just
as the non-user formats would do.

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

archive.c

@@ -48,7 +48,7 @@ static void format_subst(const struct commit *commit,
 		strbuf_add(&fmt, b + 8, c - b - 8);
 
 		strbuf_add(buf, src, b - src);
-		format_commit_message(commit, fmt.buf, buf);
+		format_commit_message(commit, fmt.buf, buf, DATE_NORMAL);
 		len -= c + 1 - src;
 		src  = c + 1;
 	}