BlockLevelPass: minor changes due to initial code review

BlockLevelPass: minor changes due to initial code review

* Improve some comments
* In getCommon() use min() to improve readability
* Use strict equals where possible
* Use camel case in variable names
* Remove the "case 0" optimisation, made sense in PHP 4 but those are
  compile-time constants now and are presumably treated as such in both
  PHP and HHVM.
* oLine -> inputLine, no idea what "o" stood for
* pos -> colonPos, lt -> ltPos: descriptive
* stack -> level, paragraphStack -> pendingPTag: they're not stacks
* Remove "m" prefix from member variable names

Change-Id: I6c1144c792ba3e1935be88a009a6d6c110d11090