Several bugs were found and fixed while getting this to work:
* Remember the 'R'(eplace) case of actions and treat it like we
would an 'A'(dd) case.
* Fix a small case of follow-parent missing a parent if a
subdirectory was modified in the revision where the parent was
copied.
* dirents returned by get_dir sometimes expire if the data
structure is too big and the pool is destroyed, so we
cache get_dir (along with check_path and get_revprops)
temporarily along with its pool.