Switch back to oh-my-zsh.hide-dirty setting again

Commit 81004dfaba509ff62a13ba303ab941938d619326 reverted the change
in 9b811fb625c03c30a766191cdf65a1c7c1fd96b2 when editing the merge
conflict from #2928.

This commit fixes that so that we don't make the same mistake again.

First seen in http://git.io/Cdaj5Q