bash: offer only paths after '--' for 'git checkout'

Commit d773c631 (bash: offer only paths after '--', 2008-07-08) did the
same for several other git commands, but 'git checkout' went unnoticed.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -626,6 +626,8 @@ _git_bundle ()
 
 _git_checkout ()
 {
+	__git_has_doubledash && return
+
 	__gitcomp "$(__git_refs)"
 }