complete: zsh: trivial simplification

There should be no functional changes.

The only reason I wrapped this code around a sub-function is because zsh
did the same in it's bashcompinit script in order to declare the special
variable 'words' as hidden, but only in this context.

There's no need for that any more since we access __git_main directly,
so 'words' is not modified, so there's no need for the sub-function.

In zsh mode the array indexes are different though.

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