histedit: fix call to _getgoal() by adding a byteskwargs() wrapper

Authored by durin42.

Description

histedit: fix call to _getgoal() by adding a byteskwargs() wrapper

I also added some b-prefixes while I was here because I got confused
and it seems silly to not just add them since it clarifies the whole
change.

Differential Revision: https://phab.mercurial-scm.org/D5585