Use RFC2822 dates from "git fmt-patch".

Still Work-in-progress git fmt-patch (should it be known as
format-patch-ng?) is matched with the fix made by Huw Davies
in 262a6ef76a1dde97ab50d79fa5cd6d3f9f125765 commit to use
RFC2822 date format.

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/commit.c b/commit.c
index f4e4eea..42b44bb 100644
--- a/commit.c
+++ b/commit.c
@@ -442,7 +442,8 @@
 		ret += sprintf(buf + ret, "Date:   %s\n", show_date(time, tz));
 		break;
 	case CMIT_FMT_EMAIL:
-		ret += sprintf(buf + ret, "Date: %s\n", show_date(time, tz));
+		ret += sprintf(buf + ret, "Date: %s\n",
+			       show_rfc2822_date(time, tz));
 		break;
 	case CMIT_FMT_FULLER:
 		ret += sprintf(buf + ret, "%sDate: %s\n", what, show_date(time, tz));