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

50c5885 (git-completion.bash: replace zsh notation that breaks bash
3.X, 2013-01-18) fixed a zsh-ism introduced earlier to append to an
array, which older versions of bash (3.0) did not grok.  This was
again broken by 734b2f0 (completion: synchronize zsh wrapper,
2013-05-08).

Cherry-pick the fix again to let those with older bash use the
completion script.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2663,7 +2663,7 @@ 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