[PATCH v4 (for maint)] git-completion: fix zsh support

It turns out 'words' is a special variable used by zsh completion. This was not isolated correctly in zsh's bash completion emulation, so by trying to set it as 'local' in git's completion, unexpected results occur; assignations are not propagated to outer levels in the call stack.

This is now fixed in the latest master branch of zsh[1] by simply defining 'words' as hidden (typeset -h), which removes the special meaning inside the emulated bash function. It probably won't be released until version 4.3.12.

In the meantime, we can workaround the issue by doing the same; defining words as hidden (typeset -h) as soon as possible.

Right now zsh is completely broken after commit da48616 (bash: get --pretty=m<tab> completion to work with bash v4), which introduced _get_comp_words_by_ref() that comes from bash-completion[2] scripts, and relies on the 'words' variable to behave like any normal variable.

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

Comments-by: SZEDER Gábor <szeder@ira.uka.de>
Comments-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
---
 contrib/completion/git-completion.bash |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

This patch is meant for the maintenance branch, so Szeder's patches are not required.

v2: fix _gitk() too as Szeder suggested.

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

v4: more commit message improvements