t0006: check --date=format zone offsets

We already test that "%z" and "%Z" show the right thing, but
we don't actually check that the time we display is the
correct one. Let's add two new tests:

  1. Test that "format:" shows the time in the author's
     timezone, just like the other time formats.

  2. Test that "format-local:" shows time in the local
     timezone. We don't want to use our normal UTC for this,
     because its offset is zero (so the result would be
     "correct" even if the code forgot to apply the offset
     or applied it in the wrong direction).

     We'll use the EST5 zone, which is already used
     elsewhere in the script (and so is assumed to be
     available everywhere).

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

t/t0006-date.sh

@@ -31,9 +31,11 @@ check_show () {
 	format=$1
 	time=$2
 	expect=$3
-	test_expect_success $4 "show date ($format:$time)" '
+	prereqs=$4
+	zone=$5
+	test_expect_success $prereqs "show date ($format:$time)" '
 		echo "$time -> $expect" >expect &&
-		test-date show:$format "$time" >actual &&
+		TZ=${zone:-$TZ} test-date show:"$format" "$time" >actual &&
 		test_cmp expect actual
 	'
 }
@@ -57,6 +59,9 @@ check_show 'format:%Z' "$TIME" ''
 check_show 'format:%%z' "$TIME" '%z'
 check_show 'format-local:%%z' "$TIME" '%z'
 
+check_show 'format:%Y-%m-%d %H:%M:%S' "$TIME" '2016-06-15 16:13:20'
+check_show 'format-local:%Y-%m-%d %H:%M:%S' "$TIME" '2016-06-15 09:13:20' '' EST5
+
 # arbitrary time absurdly far in the future
 FUTURE="5758122296 -0400"
 check_show iso       "$FUTURE" "2152-06-19 18:24:56 -0400" LONG_IS_64BIT