Fix tree name identification when >1 levels of dirs

Fixes issue reported on the forum (http://root.cern.ch/phpBB3/viewtopic.php?f=13&t=17605)
Reset fCreateSelObj after by-object run

Fixes bug affecting repeated runs with mixed by-obj and by-file processing
(for example, stressProof failures of new by-file processing tests placed after
 test #28)
Add new test #29 testing trees in sub-dirs

Address the use case reported on the forum (http://root.cern.ch/phpBB3/viewtopic.php?f=13&t=17605)