Fix crazystairs with non-stairlike reverse-indented lists

Summary: Ref T5102. This loop is supposed to select the first item at the minimum depth, but uses < rather than <= so it selects the last item instead.

Test Plan:
Added and ran unit tests. Viewed stairs in preview, they worked correctly.

{F181101}

Reviewers: asherkin, btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5102

Differential Revision: https://secure.phabricator.com/D10043