fix compile warnings (#301)

Author: Byron

git-worktree/src/lib.rs

@@ -3,7 +3,8 @@
     feature = "document-features",
     cfg_attr(doc, doc = ::document_features::document_features!())
 )]
-#![forbid(unsafe_code, rust_2018_idioms)]
+#![deny(unsafe_code)]
+#![deny(rust_2018_idioms)]
 
 /// file system related utilities
 pub mod fs;