`lib/git-hub.d/init`: Load Bash compl only in Bash

In `lib/git-hub.d/init`, load the Bash completion facilities for `git
hub` (and for `git`) only if running under Bash (a status which is
assumed to be true if the shell parameter `BASH_VERSION` is non-null).

(Note: the diff is mostly increased indentation; it might look
friendlier with `--ignore-space-change` (`-b`).)