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>