s/type -f/type -t/ to check for __gitcomp function

In `lib/git-hub.d/init`, to check for the availability of a `__gitcomp`
Bash function, use `type -t __gitcomp` rather than `type -f __gitcomp`.

From my limited testing, `type -t` appears to give the desired output,
whereas `type -f` would not.
Load Bash compl even if `__gitcomp` already exists

In `lib/git-hub.d/init`, load the Bash completion facilities for `git
hub` regardless of whether the `__gitcomp` Bash function, defined by the
standard Git completion script, was already available without
`lib/git-hub.d/init` needing to source the standard Git completion
script.

Previously, the Bash completion facilities for `git hub` would only be
loaded if `__gitcomp` was not available prior to `lib/git-hub.d/init`’s
execution, which did not seem to me to be the desired behavior.
Merge pull request #142 from 8573/8573/fix/shell-env-init-script/type-t-not-type-f/1

s/type -f/type -t/ to check for __gitcomp function
Merge pull request #143 from 8573/8573/fix/shell-env-init-script/load-bash-compl-even-if-gitcomp-already-loaded/1

Load Bash compl even if `__gitcomp` already exists
`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`).)
Merge pull request #144 from 8573/8573/fix/shell-env-init-script/load-bash-compl-only-in-bash/1

`lib/git-hub.d/init`: Load Bash compl only in Bash
Add basic zsh completion script

Add a basic zsh completion script for `git-hub` and `git hub`, at
`share/zsh-completion/_git-hub`, that, if hooked into the zsh completion
system, simply causes zsh to complete available `git-hub` subcommands
(as listed in `lib/git-hub.d/completion`).