Allow `FileFinder` to find paths which match a given path exactly.

Summary: Ref T5105. This functionality is required in order to discover all `.editorconfig` files.

Test Plan: Wrote and executed unit tests.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5105

Differential Revision: https://secure.phabricator.com/D9295