Merge branch 'ab/fsck-skiplist'

Update fsck.skipList implementation and documentation. * ab/fsck-skiplist: fsck: support comments & empty lines in skipList fsck: use oidset instead of oid_array for skipList fsck: use strbuf_getline() to read skiplist file fsck: add a performance test for skipList fsck: add a performance test fsck: document that skipList input must be unabbreviated fsck: document and test commented & empty line skipList input fsck: document and test sorted skipList input fsck tests: add a test for no skipList input fsck tests: setup of bogus commit object