contrib/completion: "local var=()" is misinterpreted as func-decl by zsh

contrib/completion: "local var=()" is misinterpreted as func-decl by zsh

Certain versions of zsh seems to treat

    local var=()

as a function declaration, rather than an assignment of an empty array,
although its documentation does not suggest that this should be the case.

With zsh 4.3.15 on Fedora Core 15, this causes

  __git_ps1 " (%s)"

to trigger an error message:

  local:2: command not found: svn_url_pattern

when GIT_PS1_SHOWUPSTREAM="auto".

Signed-off-by: Alex Merry <dev@randomguy3.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>