Merge branch 'ab/fsck-transfer-updates'

Merge branch 'ab/fsck-transfer-updates'

The test performed at the receiving end of "git push" to prevent
bad objects from entering repository can be customized via
receive.fsck.* configuration variables; we now have gained a
counterpart to do the same on the "git fetch" side, with
fetch.fsck.* configuration variables.

* ab/fsck-transfer-updates:
  fsck: test and document unknown fsck.<msg-id> values
  fsck: add stress tests for fsck.skipList
  fsck: test & document {fetch,receive}.fsck.* config fallback
  fetch: implement fetch.fsck.*
  transfer.fsckObjects tests: untangle confusing setup
  config doc: elaborate on fetch.fsckObjects security
  config doc: elaborate on what transfer.fsckObjects does
  config doc: unify the description of fsck.* and receive.fsck.*
  config doc: don't describe *.fetchObjects twice
  receive.fsck.<msg-id> tests: remove dead code