Fix tracked file paths for subdirectories

[klly14]

Summary

• remove --full-name from git ls-files so tracked paths stay relative to the supplied base_path
• add a regression test covering find_tracked_files when called from a subdirectory inside a git repo

Root cause

git ls-files --full-name always emits paths relative to the repository root. find_tracked_files then joined those repo-root-relative paths onto base_path, which doubled nested prefixes like /repo/backend/backend/... when the caller ran from a subdirectory.

Validation

• cargo test tracked_files -- --nocapture
• cargo test
• cargo fmt --check

[perryqh]

LGTM