Use vc-git to locate git repo root

- By delegating to vc-git (built-in since Emacs 23), we can often save
  traversing the directory tree, since vc-git caches the repo root.
- shell-command-to-string is a simpler way to collect command output
Explicitly use bug-reference
Remove redundant .* from regexp
Don't autoload find-file-hook addition, update docs

It's considered bad practice for a library to unilaterally modify hooks:
if this library were bundled with Emacs, it would not be expected to
auto-enable itself.

This commit makes users enable bug-reference-github explicitly, and
outlines a couple of alternative methods which might suit them.