completion: bash: fix for multiple dash commands

Otherwise options of commands like 'for-each-ref' are not completed.

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

@@ -421,7 +421,7 @@ __gitcomp_builtin ()
 	local incl="${2-}"
 	local excl="${3-}"
 
-	local var=__gitcomp_builtin_"${cmd/-/_}"
+	local var=__gitcomp_builtin_"${cmd//-/_}"
 	local options
 	eval "options=\${$var-}"