mktag tests: test verify_object() with replaced objects

mktag tests: test verify_object() with replaced objects

Add tests to demonstrate what "mktag" does in the face of replaced
objects.

There was an existing test for replaced objects fed to "mktag" added
in cc400f50112 (mktag: call "check_sha1_signature" with the
replacement sha1, 2009-01-23), but that one only tests a
commit->commit mapping. Not a mapping to a different type as like
we're also testing for here. We could remove the "mktag" test in
t6050-replace.sh now if the created tag wasn't being used by a
subsequent "fsck" test.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>