Merge branch 'jk/prune-mtime' into maint

Access to objects in repositories that borrow from another one on a
slow NFS server unnecessarily got more expensive due to recent code
becoming more cautious in a naive way not to lose objects to pruning.

* jk/prune-mtime:
sha1_file: only freshen packs once per run
sha1_file: freshen pack objects before loose
reachable: only mark local objects as recent

diff --cc cache.h
Simple merge

diff --cc sha1_file.c
Simple merge