Have traced down the issue. It feel like the original intent is to
ignore `Cargo.lock` and `target` at project root but not subdirectories.

1. The original implementation did ignore root `/Cargo.lock`.
   rust-lang#321
2. Someday one wanted to support both gitignore and hgingore's syntax
   and removed the leading slash.
   rust-lang#1247
3. Later, one found that we should not ignore `target` other than
   under root directory and added `/target` back.
   rust-lang#4099
4. It turns out that the syntax is not compatible between gitignore
   and hgignore. Therefore, one started to use hgignore special syntax
   to handle `Cargo.lock`.
   rust-lang#4342

This commit rollbacks to what original implementation tries to do.