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>
Merge branch 'js/fsck-tag-validation' into HEAD

* js/fsck-tag-validation:
  Make sure that index-pack --strict checks tag objects
  Add regression tests for stricter tag fsck'ing
  fsck: check tag objects' headers
  Make sure fsck_commit_buffer() does not run out of the buffer
  fsck_object(): allow passing object data separately from the object itself
  Refactor type_from_string() to allow continuing after detecting an error
t1450: make sure fsck detects a malformed tagger line

With "hash-object --literally", write a tag object that is not
supposed to pass one of the new checks added to "fsck", and make
sure that the new check catches the breakage.

Signed-off-by: Junio C Hamano <gitster@pobox.com>