Fix assertion error in when scrollback-push is enqueued from script

As soon as we start processing a scrollback-push readline command, we
pause execution of all other readline commands until scrollback-push
retires.  This means that we never get into a situation with two
active scrollback-push commands -- unless we are executing readline
commands via a script running "commandline -f":
since the first part of scrollback-push handling returns immediately,
the script will proceed before scrollback-push retires.

A second scrollback-push fails an assertion.  Work around that for now.
In future, scrollback-push should block when invoked by such a script,
just like it does when invoked from bindings.

src/reader.rs

@@ -3508,6 +3508,10 @@ impl<'a> Reader<'a> {
                 self.parser.libdata_mut().is_repaint = false;
             }
             rl::ScrollbackPush => {
+                if self.waiting_for_cursor_position.is_some() {
+                    // TODO: re-queue it I guess.
+                    return;
+                }
                 self.request_cursor_position(WaitingForCursorPosition::ScrollbackPush);
             }
             rl::SelfInsert | rl::SelfInsertNotFirst | rl::FuncAnd | rl::FuncOr => {

tests/checks/tmux-scrollback.fish

@@ -0,0 +1,8 @@
+#RUN: %fish %s
+#REQUIRES: command -v tmux
+
+isolated-tmux-start
+isolated-tmux send-keys 'bind ctrl-g "commandline -f scrollback-push scrollback-push"' Enter C-g
+tmux-sleep
+isolated-tmux capture-pane -p
+# CHECK: prompt 1>