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

When commit d8b4531 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>

contrib/completion/git-completion.bash

@@ -2431,7 +2431,7 @@ if [[ -n ${ZSH_VERSION-} ]]; then
 				--*=*|*.) ;;
 				*) c="$c " ;;
 				esac
-				array+=("$c")
+				array[$#array+1]="$c"
 			done
 			compset -P '*[=:]'
 			compadd -Q -S '' -p "${2-}" -a -- array && _ret=0