Make sure that index-pack --strict checks tag objects

One of the most important use cases for the strict tag object checking
is when transfer.fsckobjects is set to true to catch invalid objects
early on. This new regression test essentially tests the same code path
by directly calling 'index-pack --strict' on a pack containing an
tag object without a 'tagger' line.

Technically, this test is not enough: it only exercises a code path that
*warns*, not one that *fails*. The reason is that hash-object and
pack-objects both insist on parsing the tag objects and would fail on
invalid tag objects at this time.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>