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