make cache import include .gitignored files

Summary:
`git add -A` doesn't add ignored files by default. But *peru* doesn't have any
concept of ignored files. When peru imports a directory, it should import
eveything.

https://github.com/oconnor663/peru/issues/15

Test Plan: New unit test.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D12