Make very minor generality improvements to the scope selector

Summary:
See PHI985. I think we pretty much need to start applying language-specific rules, but we can apply at least one more relatively language-agnostic rule: don't match lines which are indented 3+ levels.

In C++, we may have symbols like this:

class X {
  public:
    int m() { ... }
}

..but I believe no mainstream language puts symbol definitions 3+ levels deep.

Also clean up some of the tab handling very slightly.

Test Plan: Tests pass, looked at some C++ code and got slightly better (but still not great) matches.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20479