Fix path autocomplete

Summary: Paths autocompleter sometimes omits /.

Test Plan:

1. Go to https://secure.phabricator.com/owners/new/.
2. Write /specs/h to path.
3. Delete the second slash resulting in /specsh.
4. Don't find /specshistorytest.html in autocomplete.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2981