mktag: allow turning off fsck.extraHeaderEntry

mktag: allow turning off fsck.extraHeaderEntry

In earlier commits mktag learned to use the fsck machinery, at which
point we needed to add fsck.extraHeaderEntry so it could be as strict
about extra headers as it's been ever since it was implemented.

But it's not nice to need to switch away from "mktag" to "hash-object"
+ manual "fsck" just because you'd like to have an extra header. So
let's support turning it off by getting "fsck.*" variables from the
config.

Pedantically speaking it's still not possible to make "mktag" behave
just like "hash-object -t tag" does, since we're unconditionally going
to check the referenced object in verify_object_in_tag(), which is our
own check, and not one that exists in fsck.c.

But the spirit of "this works like fsck" is preserved, in that if you
created such a tag with "hash-object" and did a full "fsck" on the
repository it would also error out about that invalid object, it just
wouldn't emit the same message as fsck does.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>