Revert "根据GPT说法，18年以后Rust可以去掉mod.rs"

This reverts commit 228e1d681e.

src/commands/mod.rs

@@ -0,0 +1,20 @@
+pub mod add;
+pub use add::add;
+pub mod branch;
+pub use branch::branch;
+pub mod commit;
+pub use commit::commit;
+pub mod init;
+pub use init::init;
+pub mod log;
+pub use log::log;
+pub mod merge;
+pub use merge::merge;
+pub mod remove;
+pub use remove::remove as rm;
+pub mod restore;
+pub use restore::restore;
+pub mod status;
+pub use status::status;
+pub mod switch;
+pub use switch::switch;