Search for definition after navigation

[wyuenho]

Currently when helpful is constructing the navigation button in the helpful buffer, it assumes the position of the definition searched is going to remain the same as after finding file. This assumption cannot hold if the user has an emacs-lisp-mode-hook defined that changes the buffer size, e.g. (indent-region (point-min) (point-max)), so in order to make sure the point is really pointing at the definition, helpful--navigate will have to search again.

This logic is mostly copied from help-mode, which does not have the same problem.