Fix a bug in inferring redex-all path
Fix a bug in inferring redex-all path

Summary:
In a certain case, `__file__` can be pointed to /path/fb-redex.pex/redex.pyc

Previously, dirname was called only time on `__file__` to get the directory, which in turn was concatenated with 'redex-all'.

This fix will call dirname multiple times until reaching the actual directory.

Reviewed By: int3

Differential Revision: D4720106

fbshipit-source-id: 5826f2475f5061040a0c1d8de0514412cbdcaf6d

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex