Rewrite `export_tree` to allow deleted files.

Summary:
In issue #23 we discussed allowing peru to "overwrite" deleted
imports. As part of doing this, I wanted to rewrite `export_tree` with proper
plumbing commands. I got a great StackOverflow answer from @jthill
(http://stackoverflow.com/a/24986880/823869) that pointed me to
`git read-tree -mu`.

Test Plan:
Extend a test to make sure the deleted case is covered. Make sure it
fails without this change.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D39