Test fsck a bit harder

Test fsck a bit harder

git-fsck, of all tools, has very few tests.  This adds some more:
* a corrupted object;
* a branch pointing to a non-commit;
* a tag pointing to a nonexistent object;
* and a tag pointing to an object of a type other than what the tag
  itself claims.

Only the first two are caught.  At least the third probably should,
too, but currently slips through.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>