Keep undo history across prompts

After abandoning a commandline (for example with ctrl-c) it's nice to be
able to restore it. There is little reason to discard the requisite undo
information, so keep it.

CHANGELOG.rst

@@ -60,6 +60,7 @@ Notable improvements and fixes
    Any key argument that starts with an ASCII control character (like ``\e`` or ``\cX``) or is up to 3 characters long and not a named key and does not contain ``,`` or ``-`` will be interpreted in the old syntax to keep compatibility for the majority of bindings. This should cover the majority of bindings in use.
 - A new function ``fish_should_add_to_history`` can be overridden to decide whether a command should be added to the history (:issue:`10302`).
 - :kbd:`ctrl-c` during command input no longer prints ``^C`` and a new prompt but merely clears the command line. This restores the behavior from version 2.2. To revert to the old behavior use ``bind ctrl-c __fish_cancel_commandline`` (:issue:`10213`).
+- Undo history is no longer truncated after every command but kept for the lifetime of the shell process.
 - The :kbd:`ctrl-r` history search now uses glob syntax (:issue:`10131`).
 - The :kbd:`ctrl-r` history search now operates only on the line at cursor, making it easier to quickly compose a multi-line command by recalling previous commands.
 - Abbreviations can now be restricted to specific commands. For instance::

src/editable_line.rs

@@ -160,17 +160,13 @@ impl EditableLine {
     }
 
     pub fn clear(&mut self) {
-        self.undo_history.clear();
         if self.is_empty() {
             return;
         }
-        let len = self.len();
-        apply_edit(
-            &mut self.text,
-            &mut self.colors,
-            &Edit::new(0..len, L!("").to_owned()),
+        self.push_edit(
+            Edit::new(0..self.len(), L!("").to_owned()),
+            /*allow_coalesce=*/ false,
         );
-        self.set_position(0);
     }
 
     /// Modify the commandline according to @edit. Most modifications to the

src/reader.rs

@@ -625,8 +625,8 @@ fn read_i(parser: &Parser) -> i32 {
             continue;
         }
 
-        data.update_buff_pos(EditableLineTag::Commandline, Some(0));
         data.command_line.clear();
+        data.update_buff_pos(EditableLineTag::Commandline, None);
         data.command_line_changed(EditableLineTag::Commandline);
         data.screen.write_bytes(b"\x1b]133;C\x07");
         event::fire_generic(parser, L!("fish_preexec").to_owned(), vec![command.clone()]);