Fix a fatal if a directory listing contains a non-file called 'readme'

Summary: See T1665. If you have a directory named 'readme', we try to read it as a README.

Test Plan: Created a directory named 'readme', hit a similar fatal to the one in T1665, applied this patch, everything worked great.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1665

Differential Revision: https://secure.phabricator.com/D3312