git-completion: fix regression in zsh support

git-completion: fix regression in zsh support

The zsh support of git-completion script in contrib/ is broken for current
versions of zsh, and does not notice when there's a subcommand.

For example: "git log origi<TAB>" gives no completions because it would
try to find a "git origi..." command. This will be fixed by zsh 4.3.12,
but for now we can workaround it by backporting the same fix as zsh folks
implemented.

The problem started after commit v1.7.4-rc0~11^2~2 (bash: get
--pretty=m<tab> completion to work with bash v4), which introduced
_get_comp_words_by_ref() that comes from bash-completion[1] scripts, and
relies on the 'words' variable.

However, it turns out 'words' is a special variable used by zsh
completion. From zshcompwid(1):

  [...] the parameters are reset on each function exit (including nested
  function calls from within the completion widget) to the values they had
  when the function was entered.

As a result, subcommand words are lost.  Ouch.

This is now fixed in the latest master branch of zsh[2] by simply defining
'words' as hidden (typeset -h), which removes the special meaning inside
the emulated bash function. So let's do the same.

Jonathan Nieder helped on the commit message.

  [1] http://bash-completion.alioth.debian.org/
  [2] http://zsh.git.sourceforge.net/git/gitweb.cgi?p=zsh/zsh;a=commitdiff;h=e880604f029088f32fb1ecc39213d720ae526aaa

Reported-by: Stefan Haller <lists@haller-berlin.de>
Comments-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>