Use into_owned over to_string for Cow<str> (#39024)

This removes unnecessary allocations when the `Cow` is already owned


Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/git_ui/src/text_diff_view.rs

@@ -193,7 +193,7 @@ impl TextDiffView {
             .and_then(|b| {
                 b.read(cx)
                     .file()
-                    .map(|f| f.full_path(cx).compact().to_string_lossy().to_string())
+                    .map(|f| f.full_path(cx).compact().to_string_lossy().into_owned())
             })
             .unwrap_or("untitled".into());