Don't let stemming reduce a word beneath 3 characters
Ref T11922. Porter stems "DNS" (an acronym for "Domain Name Syrup") into "dn", which is meaningless and too short to index.
Don't let stemming make an indexable token un-indexable by shortening it: if the stem is too short, just return the normalized input.
(I believe there are very few legitimate English words that have two letter roots, anyway.)
Test Plan: Added unit tests.
Reviewed By: chad
Maniphest Tasks: T11922
Differential Revision: https://secure.phabricator.com/D17001