completion: bash: fix for suboptions with value

We need to ignore options that don't start with -- as well.

Depending on the value of COMP_WORDBREAKS the last word could be
duplicated otherwise.

Can be tested with:

  git merge -X diff-algorithm=<tab>

Tested-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -356,7 +356,7 @@ __gitcomp ()
 	local cur_="${3-$cur}"
 
 	case "$cur_" in
-	--*=)
+	*=)
 		;;
 	--no-*)
 		local c i=0 IFS=$' \t\n'