t9902-completion.sh: old Bash still does not support array+=('') notation

Old Bash (3.0) which is distributed with RHEL 4.X and other ancient
platforms that are still in wide use, does not understand the
array+=() notation.  Let's use an explicit assignment to the new array
element which works everywhere, like:

   array[${#array[@]}+1]=''

The right-hand side '' is not strictly necessary, but in this case
I think it is more clear.
Signed-off-by: NBrandon Casey <drafnel@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t9902-completion.sh

@@ -69,7 +69,7 @@ run_completion ()
 	local -a COMPREPLY _words
 	local _cword
 	_words=( $1 )
-	test "${1: -1}" = ' ' && _words+=('')
+	test "${1: -1}" = ' ' && _words[${#_words[@]}+1]=''
 	(( _cword = ${#_words[@]} - 1 ))
 	__git_wrap__git_main && print_comp
 }