Merge branch 'jk/prune-optim'

"git prune" has been taught to take advantage of reachability
bitmap when able.

* jk/prune-optim:
  t5304: rename "sha1" variables to "oid"
  prune: check SEEN flag for reachability
  prune: use bitmaps for reachability traversal
  prune: lazily perform reachability traversal