Check for external file modifications when writing (#5805)

`:write` and other file-saving commands now check the file modification
time before writing to protect against overwriting external changes.

Co-authored-by: Gustavo Noronha Silva <gustavo@noronha.dev.br>
Co-authored-by: LeoniePhiline <22329650+LeoniePhiline@users.noreply.github.com>
Co-authored-by: Pascal Kuthe <pascal.kuthe@semimod.de>

helix-term/tests/test/helpers.rs

@@ -319,6 +319,12 @@ impl AppBuilder {
     }
 }
 
+pub async fn run_event_loop_until_idle(app: &mut Application) {
+    let (_, rx) = tokio::sync::mpsc::unbounded_channel();
+    let mut rx_stream = UnboundedReceiverStream::new(rx);
+    app.event_loop_until_idle(&mut rx_stream).await;
+}
+
 pub fn assert_file_has_content(file: &mut File, content: &str) -> anyhow::Result<()> {
     file.flush()?;
     file.sync_all()?;