fixup! fsck: inspect tag objects more closely
Refactor out fsck_object_buffer() accepting the object data

When fsck'ing an incoming pack, we need to fsck objects that cannot be
read via read_sha1_file() because they are not local yet (and might even
be rejected if transfer.fsckobjects is set to 'true').

For commits, there is a hack in place: we basically cache commit
objects' buffers anyway, but the same is not true for tag objects.

By refactoring fsck_object() to take the object buffer as an optional
argument, we prepare the machinery for the upcoming handling of tag
objects.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
fixup! fsck: inspect tag objects more closely
fixup! Refactor out fsck_object_buffer() accepting the object data
fixup! Make sure that index-pack --strict fails upon invalid tag objects