init: Search more thoroughly for Git compl script

Previously, in `lib/git-hub.d/init`, when executing under Bash, the
standard Git completion script for Bash was checked for only at the
file-path `/etc/bash_completion.d/git`.

None of my computing devices (whether they run Gentoo Linux, Ubuntu
Linux, Mac OS X (with MacPorts), or — going on an assumption here —
Android) have a `/etc/bash_completion.d/git` file; however, Ingy döt
Net’s Ubuntu system reportedly does have such a file.

Having found, in a Git completion script for zsh (that’s in Git’s
repository, but is different from the standard Git completion script for
zsh that’s in zsh’s repository), a list of file-paths whereat the
standard Git completion script for Bash might be found [1], and also
having found the location of the standard Git completion script for Bash
on my Gentoo, Ubuntu, and Mac systems —

 - `/usr/share/bash-completion/git` (Gentoo),
 - `/usr/share/bash-completion/completions/git` (Ubuntu),
 - `/opt/local/share/bash-completion/completions/git` (MacPorts)

— I’ve extended the section of `lib/git-hub.d/init` concerned with
finding and loading, if necessary, the standard Git completion script
for Bash such that it should check for that script at all the above
file-paths.

[1]: <https://github.com/git/git/blob/4109c28e055dba27d73cefb956bea5e611f66ec0/contrib/completion/git-completion.zsh#L34-L36>