completion: reflog show <log-options>

Let's add completion for <log-options> in "reflog show" so that the user
can easily discover uses like:

   $ git reflog --since=1.day.ago

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2407,6 +2407,16 @@ _git_rebase ()
 _git_reflog ()
 {
 	local subcommands="show delete expire"
+	local subcommand="$(__git_find_subcommand "$subcommands" "show")"
+
+	case "$subcommand,$cur" in
+	show,--*)
+		__gitcomp "
+			$__git_log_common_options
+			"
+		return
+		;;
+	esac
 
 	__git_complete_refs