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