fsck: inspect tag objects more closely

We inspect commit objects pretty much in detail in git-fsck, but we just
glanced over the tag objects. Let's be stricter.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add regression tests for stricter tag fsck'ing

The intent of the two new test cases is to catch general breakages in
the fsck_tag() function, not so much to test it extensively, trying to
strike the proper balance between thoroughness and speed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>