DirCacheIterator: Fix reset() and back()

DirCacheIterator: Fix reset() and back()

reset() was broken and probably only worked when the position was
at the beginning. More serious was that back() sometimes descended
into the tree rather than skipping backward at the same level. Sometimes
this would result in false conflicts, but one could suspect silent
errors too. back() is called by the NamingConflictTreeWalk when looking
for directory/file conflicts.

Also added toString to DirCacheTree to simplify debugging.

Bug: 396127
Change-Id: Iaa1b4e20e623d84c2e5ac26748f42e991080dbcd