complete: zsh: use zsh completion for the main cmd

So that we can have a nice zsh completion output:

% git <tab>
add       -- add file contents to the index
bisect    -- find by binary search the change that introduced a bug
branch    -- list, create, or delete branches
checkout  -- checkout a branch or paths to the working tree
clone     -- clone a repository into a new directory
commit    -- record changes to the repository
diff      -- show changes between commits, commit and working tree, etc
fetch     -- download objects and refs from another repository
grep      -- print lines matching a pattern
init      -- create an empty Git repository or reinitialize an existing one
log       -- show commit logs
merge     -- join two or more development histories together
mv        -- move or rename a file, a directory, or a symlink
pull      -- fetch from and merge with another repository or a local branch
push      -- update remote refs along with associated objects
rebase    -- forward-port local commits to the updated upstream head
reset     -- reset current HEAD to the specified state
rm        -- remove files from the working tree and from the index
show      -- show various types of objects
status    -- show the working tree status
tag       -- create, list, delete or verify a tag object signed with GPG

And other niceties, like 'git --git-dir=<tab>' showing only directories.

For the rest, the bash completion stuff is still used.

Also, add my copyright, since this more than a thin wrapper.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>