git-completion.bash: replace zsh notation that breaks bash 3.X

When commit d8b45314 began separating the zsh completion from the bash completion, it introduced a zsh completion "bridge" section into the bash completion script for zsh users to use until they migrated to the zsh script. The zsh '+=()' append-to-array notation prevents bash 3.00.15 on CentOS 4.x from loading the completion script and breaks test 9902. We can easily work around this by using standard Bash array notation. Signed-off-by: Brandon Casey <drafnel@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>