diff: ignore sparse paths in diffstat

The diff_populate_filespec() method is used to describe the diff after a
merge operation is complete, especially when a conflict appears. In
order to avoid expanding a sparse index, the reuse_worktree_file() needs
to be adapted to ignore files that are outside of the sparse-checkout
cone. The file names and OIDs used for this check come from the merged
tree in the case of the ORT strategy, not the index, hence the ability
to look into these paths without having already expanded the index.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

diff.c

@@ -3992,6 +3992,13 @@ static int reuse_worktree_file(struct index_state *istate,
 	if (!FAST_WORKING_DIRECTORY && !want_file && has_object_pack(oid))
 		return 0;
 
+	/*
+	 * If this path does not match our sparse-checkout definition,
+	 * then the file will not be in the working directory.
+	 */
+	if (!path_in_sparse_checkout(name, istate))
+		return 0;
+
 	/*
 	 * Similarly, if we'd have to convert the file contents anyway, that
 	 * makes the optimization not worthwhile.