Don't allow clear-history to run if pane is in a mode

gistfile1.txt

diff --git a/cmd-clear-history.c b/cmd-clear-history.c
index cce3ea1..8b16215 100644
--- a/cmd-clear-history.c
+++ b/cmd-clear-history.c
@@ -44,6 +44,10 @@ cmd_clear_history_exec(struct cmd *self, struct cmd_q *cmdq)

        if (cmd_find_pane(cmdq, args_get(args, 't'), NULL, &wp) == NULL)
                return (CMD_RETURN_ERROR);
+
+       if (wp->mode == &window_copy_mode)
+               return (CMD_RETURN_ERROR);
+
        gd = wp->base.grid;

        grid_move_lines(gd, 0, gd->hsize, gd->sy);