Make show_rfc2822_date() just another date output format.

These days, show_date() takes a date_mode parameter to specify
the output format, and a separate specialized function for dates
in E-mails does not make much sense anymore.

This retires show_rfc2822_date() function and make it just
another date output format.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/refs.c b/refs.c
index 4dc7e8b..2694e70 100644
--- a/refs.c
+++ b/refs.c
@@ -1300,7 +1300,7 @@
 				if (hashcmp(logged_sha1, sha1)) {
 					fprintf(stderr,
 						"warning: Log %s has gap after %s.\n",
-						logfile, show_rfc2822_date(date, tz));
+						logfile, show_date(date, tz, DATE_RFC2822));
 				}
 			}
 			else if (date == at_time) {
@@ -1313,7 +1313,7 @@
 				if (hashcmp(logged_sha1, sha1)) {
 					fprintf(stderr,
 						"warning: Log %s unexpectedly ended on %s.\n",
-						logfile, show_rfc2822_date(date, tz));
+						logfile, show_date(date, tz, DATE_RFC2822));
 				}
 			}
 			munmap(log_mapped, mapsz);