Various fixes to codepaths gcc 9 had trouble following dataflow.

* jk/misc-uninitialized-fixes:
  pack-objects: drop packlist index_pos optimization
  test-read-cache: drop namelen variable
  diff-delta: set size out-parameter to 0 for NULL delta
  bulk-checkin: zero-initialize hashfile_checkpoint
  pack-objects: use object_id in packlist_alloc()
  git-am: handle missing "author" when parsing commit