Fix handling of --diff flag (#33094)

* Restore the ability to combine --diff with other path arguments
* Restore combining --diff with --wait

There is still one defect in the current handling of `--diff`: when Zed
is already open, we'll open the diff view in your current active zed
window. It would be better to search all of the open zed windows for any
window containing the diffed paths, but implementing that is a bit
complex. Currently, the logic for *picking* an existing zed window is
coupled to the logic for opening buffers in that window. I'd like to
decouple it, but I wanted to keep this change small, so that we hotfix
it to stable without too much risk.

Release Notes:

- Fixed a bug where the `--diff` CLI flag did not work with `--wait`

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/zed/src/zed/open_listener.rs

@@ -296,7 +296,7 @@ pub async fn handle_cli_connection(
                 env,
                 user_data_dir: _,
             } => {
-                if !urls.is_empty() || !diff_paths.is_empty() {
+                if !urls.is_empty() {
                     cx.update(|cx| {
                         match OpenRequest::parse(RawOpenRequest { urls, diff_paths }, cx) {
                             Ok(open_request) => {
@@ -346,7 +346,7 @@ async fn open_workspaces(
     env: Option<collections::HashMap<String, String>>,
     cx: &mut AsyncApp,
 ) -> Result<()> {
-    let grouped_locations = if paths.is_empty() {
+    let grouped_locations = if paths.is_empty() && diff_paths.is_empty() {
         // If no paths are provided, restore from previous workspaces unless a new workspace is requested with -n
         if open_new_workspace == Some(true) {
             Vec::new()