fetch: implement fetch.fsck.*

fetch: implement fetch.fsck.*

Implement support for fetch.fsck.* corresponding with the existing
receive.fsck.*. This allows for pedantically cloning repositories with
specific issues without turning off fetch.fsckObjects.

One such repository is https://github.com/robbyrussell/oh-my-zsh.git
which before this change will emit this error when cloned with
fetch.fsckObjects:

    error: object 2b7227859263b6aabcc28355b0b994995b7148b6: zeroPaddedFilemode: contains zero-padded file modes
    fatal: Error in object
    fatal: index-pack failed

Now with fetch.fsck.zeroPaddedFilemode=warn we'll warn about that
issue, but the clone will succeed:

    warning: object 2b7227859263b6aabcc28355b0b994995b7148b6: zeroPaddedFilemode: contains zero-padded file modes
    warning: object a18c4d13c2a5fa2d4ecd5346c50e119b999b807d: zeroPaddedFilemode: contains zero-padded file modes
    warning: object 84df066176c8da3fd59b13731a86d90f4f1e5c9d: zeroPaddedFilemode: contains zero-padded file modes

The motivation for this is to be able to turn on fetch.fsckObjects
globally across a fleet of computers but still be able to manually
clone various legacy repositories by either white-listing specific
issues, or better yet whitelist specific objects.

The use of --git-dir=* instead of -C in the tests could be considered
somewhat archaic, but the tests I'm adding here are duplicating the
corresponding receive.* tests with as few changes as possible.

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