reader: fix regressions when moving between lines

Fixes some regressions from 35ca42413 ("Simplify some parse_util functions").
The tmux tests are not beautiful but I find them easy to write.
Probably a pexpect test would also be enough here?

tests/checks/tmux-bind.fish

@@ -0,0 +1,36 @@
+#RUN: %fish -C 'set -g fish %fish' %s
+#REQUIRES: command -v tmux
+
+set fish (builtin realpath $fish)
+
+# Isolated tmux.
+set -g tmpdir (mktemp -d)
+
+# Don't CD elsewhere, because tmux socket file is relative to CWD. Using
+# absolute path to socket file is prone to 'socket file name too long' error.
+cd $tmpdir
+
+set -g tmux tmux -S .tmux-socket -f /dev/null
+
+set -g sleep sleep .1
+set -q CI && set sleep sleep 1
+
+
+$tmux new-session -x 80 -y 10 -d $fish -C '
+    # This is similar to "tests/interactive.config".
+    function fish_greeting; end
+    function fish_prompt; printf "prompt $status_generation> "; end
+    # No autosuggestion from older history.
+    set fish_history ""
+'
+$sleep # Let fish draw a prompt.
+
+# Test moving around with up-or-search on a multi-line commandline.
+$tmux send-keys 'echo 12' M-Enter 'echo ab' C-p 345 C-n cde
+$sleep
+$tmux capture-pane -p
+# CHECK: prompt 0> echo 12345
+# CHECK: echo abcde
+
+$tmux kill-server
+rm -r $tmpdir