Refactor git repository initialization and enhance logging levels

- Updated the `flatten_tree` function in `git.rs` to clarify the repository parameter name.
- Changed logging level from `debug` to `trace` in `finalize_patch_application` for more detailed output.
- Modified the `TestServeSession` initialization to persist git configuration to disk, ensuring user identity is accessible for subsequent operations.

These changes aim to improve code clarity, enhance logging detail, and ensure proper git configuration handling in tests.

src/snapshot/patch_apply.rs

@@ -145,7 +145,7 @@ fn finalize_patch_application(context: PatchApplyContext, tree: &mut RojoTree) -
     for (id, map) in context.path_refs_to_rewrite {
         for (prop_name, path) in map {
             if let Some(target) = tree.resolve_ref_path(&path, id) {
-                log::debug!(
+                log::trace!(
                     "Resolved path reference {} -> {} (path: {})",
                     prop_name,
                     target,