refactor-864370: Allow low cost prefix matches in spell.js
refactor-864370: Allow low cost prefix matches in spell.js

If the user has typed "re" then it's more likely that they are going to
want "readme.txt" than "xx" which would otherwise be closer according to
DL. So this adjusts to reduce the cost of prefix matches.

Signed-off-by: Joe Walker <7a17872fbb32c7d760c9a16ee3076daead11efcf@mozilla.com>

mozilla/gcli,joewalker/gcli,mozilla/gcli,joewalker/gcli,mozilla/gcli