t4212: test bogus timestamps with git-log

When t4212 was originally added by 9dbe7c3d (pretty: handle
broken commit headers gracefully, 2013-04-17), it tested our
handling of commits with broken ident lines in which the
timestamps could not be parsed. It does so using a bogus line
like "Name <email>-<> 1234 -0000", because that simulates an
error that was seen in the wild.

Later, 03818a4a (split_ident: parse timestamp from end of
line, 2013-10-14) made our parser smart enough to actually
find the timestamp on such a line, and t4212 was adjusted to
match. While it's nice that we handle this real-world case,
this meant that we were not actually testing the
bogus-timestamp case anymore.

This patch adds a test with a totally incomprehensible
timestamp to make sure we are testing the code path.

Note that the behavior is slightly different between regular log
output and "--format=%ad". In the former case, we produce a
sentinel value and in the latter, we produce an empty
string. While at first this seems unnecessarily
inconsistent, it matches the original behavior given by
9dbe7c3d.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t4212-log-corrupt.sh

@@ -39,4 +39,25 @@ test_expect_success 'git log --format with broken author email' '
 	test_cmp expect.err actual.err
 '
 
+munge_author_date () {
+	git cat-file commit "$1" >commit.orig &&
+	sed "s/^\(author .*>\) [0-9]*/\1 $2/" <commit.orig >commit.munge &&
+	git hash-object -w -t commit commit.munge
+}
+
+test_expect_success 'unparsable dates produce sentinel value' '
+	commit=$(munge_author_date HEAD totally_bogus) &&
+	echo "Date:   Thu Jan 1 00:00:00 1970 +0000" >expect &&
+	git log -1 $commit >actual.full &&
+	grep Date <actual.full >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'unparsable dates produce sentinel value (%ad)' '
+	commit=$(munge_author_date HEAD totally_bogus) &&
+	echo >expect &&
+	git log -1 --format=%ad $commit >actual
+	test_cmp expect actual
+'
+
 test_done