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
Merge commit 'b16e1e3' into develop