bash completion: add doubledash to "git show"

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1410,6 +1410,8 @@ _git_shortlog ()
 
 _git_show ()
 {
+	__git_has_doubledash && return
+
 	local cur="${COMP_WORDS[COMP_CWORD]}"
 	case "$cur" in
 	--pretty=*)