show-branch: use DATE_RELATIVE instead of magic number

This is more readable, and won't break if we ever change the
order of the date_mode enum.

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

builtin/show-branch.c

@@ -784,7 +784,7 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 			else
 				msg++;
 			reflog_msg[i] = xstrfmt("(%s) %s",
-						show_date(timestamp, tz, 1),
+						show_date(timestamp, tz, DATE_RELATIVE),
 						msg);
 			free(logmsg);
 			sprintf(nth_desc, "%s@{%d}", *av, base+i);