Disallow creating files with '..' (#3156)

Release Notes:

- Fixed a crash that could occur when creating files with '..' in the
path

crates/collab/src/tests/integration_tests.rs

@@ -4936,10 +4936,10 @@ async fn test_project_symbols(
         .await
         .unwrap();
 
-    buffer_b_2.read_with(cx_b, |buffer, _| {
+    buffer_b_2.read_with(cx_b, |buffer, cx| {
         assert_eq!(
-            buffer.file().unwrap().path().as_ref(),
-            Path::new("../crate-2/two.rs")
+            buffer.file().unwrap().full_path(cx),
+            Path::new("/code/crate-2/two.rs")
         );
     });