Resolve conflict between GNU cp and git cherry-pick alias

If GNU coreutils are installed as `gmv`, `gcp`, etc. via Homebrew, and `cp` is aliased to
`gcp` as is Homebrew's wont, then the tab completion for `cp` will fail oddly when the
git plugin is enabled, since zsh is completing for `git cherry-pick` but executing
`gcp`.

Being as I use `cp` much more frequently than `git cherry-pick`, I'd propose either the
cherry-pick alias be changed to `gchp` (this pull request), or removed from the plugin
and handled per-user under `/custom`.