cmd/guru: emacs: push mark in go-guru-definition

This allows users to swap point and mark to jump between use and
definition in the same buffer, or pop-tag-mark (M-*) to jump back to the
use even between multiple buffers.

This mirrors the behaviour of godef-jump from go-mode.

Also remove an obosolete TODO and fix a typo.

Change-Id: I614090493b565acadf359ebb4c7092f4c673fd56
Reviewed-on: https://go-review.googlesource.com/20303
Reviewed-by: Alan Donovan <adonovan@google.com>
1 file changed