Plugin globs do not respect .gitignore

[nathankleyn]

Currently, any globs specified in a plugin traverse into directories that are excluded by .gitignore files. This can lead to massive slowdowns, especially when deeply nested build folders are concerned.

As an example, consider the following .gitignore file:

build

Imagine this directory contains lots of files called foo in a deeply nested structure beneath it. Now imagine the following plugin:

class TestPlugin < Plugin
  projects '**/foo'
end

Despite the intentions of the repository owner, this plugin would locate the foo files stored in the build folder even though they are clearly not intended to be treated as projects. In addition, this would be potentially massively slower as we are traversing into an entire directory needlessly.

Given this, we should have some way of respecting .gitignore rules when executing globs for any reason.

[nathankleyn]

This will share the same solution as #13, which is to use a project I've been working on at nathankleyn/gitignore.rb. This allows us to get back just the tree of files that are not ignored upon which globs can be executed without touching the disk again.