Revert "根据GPT说法，18年以后Rust可以去掉mod.rs"

This reverts commit 228e1d681e.

src/models/mod.rs

@@ -0,0 +1,11 @@
+pub mod blob;
+pub use blob::Blob;
+pub mod commit;
+pub use commit::Commit;
+pub mod index;
+pub use index::FileMetaData;
+pub use index::Index;
+pub mod object;
+pub use object::Hash;
+pub mod tree;
+pub use tree::Tree;