.gitignore whitelists are not respected.

[akgerber]

This may be best explained with a simple example:
Given this .gitignore:

#start by ignoring everything
*
#and whitelist the stuff we want to keep
!problems/
!problems/*

And this directory structure:

.
└── problems
    └── fake_file.rb

where fake_file.rb contains

HELLO WORLD

ag WORLD returns no results.
ag -a WORLD returns results:

$ag -a WORLD
problems/fake_file.rb
1:HELLO WORLD

[eevee]

Duplicate of #168?

[ggreer]

Yes, this is a dupe of #168. Please watch that issue.