git-completion: workaround zsh COMPREPLY bug

git-completion: workaround zsh COMPREPLY bug

zsh adds a backslash (foo\ ) for each item in the COMPREPLY array if IFS
doesn't contain spaces. This issue has been reported[1], but there is no
solution yet.

This wasn't a problem due to another bug[2], which was fixed in zsh
version 4.3.12. After this change, 'git checkout ma<tab>' would resolve
to 'git checkout master\ '.

Aditionally, the introduction of __gitcomp_nl in commit a31e626
(completion: optimize refs completion) in git also made the problem
apparent, as Matthieu Moy reported.

The simplest and most generic solution is to hide all the changes we do
to IFS, so that "foo \nbar " is recognized by zsh as "foo bar". This
works on versions of git before and after the introduction of
__gitcomp_nl (a31e626), and versions of zsh before and after 4.3.12.

Once zsh is fixed, we should conditionally disable this workaround to
have the same benefits as bash users.

[1] http://www.zsh.org/mla/workers/2012/msg00053.html
[2] http://zsh.git.sourceforge.net/git/gitweb.cgi?p=zsh/zsh;a=commitdiff;h=2e25dfb8fd38dbef0a306282ffab1d343ce3ad8d

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>