Merge branch 'jk/prune-mtime' into maint

Merge branch 'jk/prune-mtime' into maint

The helper used to iterate over loose object directories to prune
stale objects did not closedir() immediately when it is done with a
directory--a callback such as the one used for "git prune" may want
to do rmdir(), but it would fail on open directory on platforms
such as WinXP.

* jk/prune-mtime:
  prune: close directory earlier during loose-object directory traversal