Fix --sort=ext
Fix --sort=ext

The logic of the previous version wasn't correct. Also, presuming
natural ordering of full filenames is still reasonable when the
extensions are identical.

mneumann/exa,ogham/exa,rhysd/exa,rhysd/exa,kbknapp/exa,kbknapp/exa,mneumann/exa,ogham/exa