[PATCH v5 (for maint)] git-completion: fix regression in zsh support

Right now zsh is quite broken; the completion doesn't 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. The issue would be gone in zsh 4.3.12, but for now we can workaround it by porting the same fix.

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.

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>
---
 contrib/completion/git-completion.bash |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

v2: fix _gitk() too as Szeder suggested.

v3: improve commit message as Jonathan Nieder suggested.
    Also, improve comments.

v4: more commit message improvements

v5: even more commit message improvements from Jonathan's version