diff_edit: do not make directories readonly

Otherwise directories containing files cannot be removed.

I think making files readonly is enough to tell user that the left side
cannot be edited, but I haven't tested that with Meld. If that's not enough,
we'll probably need to undo set_readonly_recursively() at exit.

src/diff_edit.rs

@@ -82,14 +82,18 @@ fn check_out(
 }
 
 fn set_readonly_recursively(path: &Path) -> Result<(), std::io::Error> {
+    // Directory permission is unchanged since files under readonly directory cannot
+    // be removed.
     if path.is_dir() {
         for entry in path.read_dir()? {
             set_readonly_recursively(&entry?.path())?;
         }
-    }
+        Ok(())
+    } else {
         let mut perms = std::fs::metadata(path)?.permissions();
         perms.set_readonly(true);
         std::fs::set_permissions(path, perms)
+    }
 }
 
 pub fn edit_diff(