fsck: rename and touch up init_skiplist()

fsck: rename and touch up init_skiplist()

init_skiplist() took a file consisting of SHA-1s and comments and added
the objects to an oidset.  This functionality is useful for other
commands and will be moved to oidset.c in a future commit.

In preparation for that move, this commit renames it to
oidset_parse_file() to reflect its more generic usage and cleans up a
few of the names.

Signed-off-by: Barret Rhoden <brho@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>