Require save confirmation and prevent autosave for deleted files (#20742)

* `has_conflict` will now return true if the file has been deleted on
disk.  This is for treating multi-buffers as conflicted, and also
blocks auto-save.

* `has_deleted_file` is added so that the single-file buffer save can
specifically mention the delete conflict. This does not yet handle
discard (#20745).

Closes #9101
Closes #9568
Closes #20462

Release Notes:

- Improved handling of externally deleted files: auto-save will be
disabled, multibuffers will treat this as a save conflict, and single
buffers will ask for restore confirmation.

Co-authored-by: Conrad <conrad@zed.dev>

crates/diagnostics/src/diagnostics.rs

@@ -727,6 +727,10 @@ impl Item for ProjectDiagnosticsEditor {
         self.excerpts.read(cx).is_dirty(cx)
     }
 
+    fn has_deleted_file(&self, cx: &AppContext) -> bool {
+        self.excerpts.read(cx).has_deleted_file(cx)
+    }
+
     fn has_conflict(&self, cx: &AppContext) -> bool {
         self.excerpts.read(cx).has_conflict(cx)
     }