git: enable zsh completion

Git ships with a zsh completion script, but this script was previously
only available at $out/share/git/contrib/completion/git-completion.zsh,
which is not a path (or a filename) that would be discovered by a
typical zsh installation. This commit symlinks that file to
$out/share/zsh/site-functions/_git, which is a more standard location.

That zsh completion script is mostly a wrapper around the Bash
completion script, so this commit also patches the former so that it can
"find" the latter.