Commit an undo checkpoint before each write (helix-editor#11062)

This fixes the modification indicator when saving from insert mode with
a config such as

    [keys.insert]
    C-s = ":write"

Previously the modification indicator would be stuck showing modified
even if the buffer contents matched the disk contents when writing after
some changes in insert mode with this binding. In insert mode we do not
eagerly write undo checkpoints so that all changes made become one
checkpoint as you exit insert mode. When saving, `Document`s `changes`
`ChangeSet` would be non-empty and when there are changes we show the
buffer as modified. Then switching to normal mode would append the
changes to history, bumping the current revision past what it was when
last saved. Since the last saved revision and current revision were then
unsynced, the modification indicator would always show modified.

This matches [Kakoune's behavior]. Kakoune has a different architecture
for writes but a very similar system for history, transactions and undo
checkpoints (what it calls "undo groups"). Upon saving Kakoune creates
an undo checkpoint if there are any uncommitted changes. It does this
after the write has gone through since its writing system is different.
For our writing system it's cleaner to make the undo checkpoint before
performing the save so that the history revision increments before we
send the save event.

[Kakoune's behavior]: https://github.com/mawww/kakoune/blob/80fcfebca8c62ace6cf2af9487784486af07d2d5/src/buffer.cc#L565-L566

helix-term/src/commands/typed.rs

@@ -340,9 +340,12 @@ fn write_impl(
     let path = path.map(AsRef::as_ref);
 
     if config.insert_final_newline {
-        insert_final_newline(doc, view);
+        insert_final_newline(doc, view.id);
     }
 
+    // Save an undo checkpoint for any outstanding changes.
+    doc.append_changes_to_history(view);
+
     let fmt = if config.auto_format {
         doc.auto_format().map(|fmt| {
             let callback = make_format_callback(
@@ -367,13 +370,12 @@ fn write_impl(
     Ok(())
 }
 
-fn insert_final_newline(doc: &mut Document, view: &mut View) {
+fn insert_final_newline(doc: &mut Document, view_id: ViewId) {
     let text = doc.text();
     if line_ending::get_line_ending(&text.slice(..)).is_none() {
         let eof = Selection::point(text.len_chars());
         let insert = Transaction::insert(text, &eof, doc.line_ending.as_str().into());
-        doc.apply(&insert, view.id);
-        doc.append_changes_to_history(view);
+        doc.apply(&insert, view_id);
     }
 }
 
@@ -704,11 +706,15 @@ pub fn write_all_impl(
 
     for (doc_id, target_view) in saves {
         let doc = doc_mut!(cx.editor, &doc_id);
+        let view = view_mut!(cx.editor, target_view);
 
         if config.insert_final_newline {
-            insert_final_newline(doc, view_mut!(cx.editor, target_view));
+            insert_final_newline(doc, target_view);
         }
 
+        // Save an undo checkpoint for any outstanding changes.
+        doc.append_changes_to_history(view);
+
         let fmt = if config.auto_format {
             doc.auto_format().map(|fmt| {
                 let callback = make_format_callback(