[HelpCommand] reset command definition if instance of self

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes 
Todo: squash if accepted
License of the code: MIT

When running ```php app/console help help```, it displays an extra ```command``` argument at the beginning.

It's because application definition is getting merged into help command (when running it the first time), then, when help command is retrieving "help", application returns the same object (with this extra argument).

Resetting command configuration in that special case removes the extra arg.