fsck: rename "skiplist" to "skip_oids"

fsck: rename "skiplist" to "skip_oids"

The "skiplist" field in "fsck_options" is related to objects. Because we
are going to introduce ref consistency check, the "skiplist" name is too
general which will make the caller think "skiplist" is related to both
the refs and objects.

It may seem that for both refs and objects, we should provide a general
"skiplist" here. However, the type for "skiplist" is `struct oidset`
which is totally unsuitable for refs.

To avoid above ambiguity, rename "skiplist" to "skip_oids".

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>