Merge branch 'jc/reflog'

* jc/reflog:
  reflog --fix-stale: do not check the same trees and commits repeatedly.
  reflog expire --fix-stale
  Move traversal of reachable objects into a separate library.
  builtin-prune: separate ref walking from reflog walking.
  builtin-prune: make file-scope static struct to an argument.

diff --cc Makefile
Simple merge