Implement files_at_path, directories_at_path, and stringify + write tests

Do you have the latest RSpec gem? There shouldn't be such inconsistency between us as I can definitely run the specs fine with just the `ENV['RAILS_ENV'] ||= 'test'` line. Try
```
bundle install
bundle exec rspec spec/lib/repo/git_revision_spec.rb
```
with the original setup. Also try running just `rspec`, which runs all the specs. I'm getting errors like this:
```
Failure/Error: revision = repo.get_latest_revision
NoMethodError:
  undefined method `target' for nil:NilClass
# ./lib/repo/git_repository.rb:609:in `latest_revision_number'
# ./lib/repo/git_repository.rb:211:in `get_latest_revision'
# ./spec/lib/repo/git_revision_spec.rb:47:in `block (4 levels) in <top (required)>'
```
Seems to be related to the issue of initialization code not inside a `let` or `before` block.