libtracker-miner: Process subdirectories only when contents are available

The leveled notification had a bug when dealing with directories at
the bottom of the crawler search results. They were being processed for
deleted files when their contents were not available because the crawler
never searched any deeper.

This commit adjusts the traversal function so that those directories
are only scanned when it is their turn as the current root.
Merge pull request #29 from amtep/master

libtracker-miner: Process subdirectories only when contents are available