bug#9438: grep regressions

[Juri Linkov <juri@jurta.org>]

> Beside `grep-error-screen-columns', I discovered two more unused defcustoms
> in grep.el:
>
>   (defcustom grep-window-height nil
>     "*Number of lines in a grep window.  If nil, use `compilation-window-height'."
>     :type '(choice (const :tag "Default" nil)
>                    integer)
>     :version "22.1"
>     :group 'grep)
>
>   (defcustom grep-scroll-output nil
>     "*Non-nil to scroll the *grep* buffer window as output appears.
>   Setting it causes the grep commands to put point at the end of their
>   output window so that the end of the output is always visible rather
>   than the begining."
>     :type 'boolean
>     :version "22.1"
>     :group 'grep)
>
> IIUC, since the docstring says "If nil, use `compilation-window-height'"
> they should be used in `grep-mode' only when the value is non-nil like:
>
>   (when grep-window-height
>     (set (make-local-variable 'compilation-window-height)
>          grep-window-height))
>   (when grep-scroll-output
>     (set (make-local-variable 'compilation-scroll-output)
>          grep-scroll-output))

According to bzr logs, before revno:54335 this used to be:

  (if grep-window-height
      (set (make-local-variable 'compilation-window-height)
	   grep-window-height))
  (set (make-local-variable 'compilation-scroll-output)
       grep-scroll-output)

So either we should restore this code in `grep-process-setup' to use
the customized values of defcustoms, or remove unused defcustoms.
I have no settled opinion what would be better to do.