fsck: reject misconfigured fsck.skipList

fsck: reject misconfigured fsck.skipList

In Git, fsck operations can ignore known broken objects via the
`fsck.skipList` configuration. This option expects a path to a file with
the list of object names. When the configuration is specified without a
path, an error message is printed, but the command continues as if the
configuration was not set. Configuring `fsck.skipList` without a value
is a misconfiguration so config parsing should be more strict and reject
it.

Update `git_fsck_config()` to no longer ignore misconfiguration of
`fsck.skipList`. The same behavior is also present for
`fetch.fsck.skipList` and `receive.fsck.skipList` so the configuration
parsers for these are updated to ensure the related operations remain
consistent.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>