Add a test for 01c8fd8 (but it's not testing set_len())

Note that this test doesn't actually validate the newly added line as
currently the file will be truncated when opening it.
When that is disabled, the test will actually kick in, and validate the safeguard.
Let's keep it for that purpose alone.

Author: Byron

gix-worktree-state/tests/state/checkout.rs

@@ -710,3 +710,38 @@ fn setup_filter_pipeline(opts: &mut gix_filter::pipeline::Options) {
         required: true,
     }];
 }
+
+#[test]
+fn checkout_truncates_existing_longer_files() -> crate::Result {
+    let mut opts = opts_from_probe();
+    opts.overwrite_existing = false;
+    opts.destination_is_initially_empty = false;
+
+    // Use existing fixture and modify one file to be longer
+    let (_source_tree, destination, _index, _outcome) = checkout_index_in_tmp_dir_opts(
+        opts.clone(),
+        "make_mixed_without_submodules_and_symlinks",
+        None,
+        |_| true,
+        |dest| {
+            // Create a longer version of the "executable" file before checkout
+            let file_path = dest.join("executable");
+            std::fs::create_dir_all(dest)?;
+            std::fs::write(
+                &file_path,
+                b"This is much longer content that should be truncated to match git's version",
+            )?;
+            Ok(())
+        },
+    )?;
+
+    let file_path = destination.path().join("executable");
+    let final_content = std::fs::read(&file_path)?;
+    assert_eq!(
+        final_content[..].as_bstr(),
+        "content",
+        "File content should match git's version"
+    );
+
+    Ok(())
+}