Merge branch 'ab/fsck-skiplist'

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