git: Install zsh and tcsh shell completion scripts to HOMEBREW/etc/bash_completion.d

`git-completion.zsh` and `git-competion.tcsh` completion scripts were recently added to the [git/contrib directory](https://github.com/git/git/tree/master/contrib/completion).

Install them to HOMEBREW/etc/bash_completion.d directory along with the `git-prompt.sh` and `git-completion.bash`.