git-completion.bash: use correct Bash/Zsh array length syntax

The syntax for retrieving the number of elements in an array is:

   ${#name[@]}

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

contrib/completion/git-completion.bash

@@ -2580,7 +2580,7 @@ if [[ -n ${ZSH_VERSION-} ]]; then
 				--*=*|*.) ;;
 				*) c="$c " ;;
 				esac
-				array[$#array+1]="$c"
+				array[${#array[@]}+1]="$c"
 			done
 			compset -P '*[=:]'
 			compadd -Q -S '' -p "${2-}" -a -- array && _ret=0