auto merge of #563 : mbrubeck/cargo/list-files-git, r=alexcrichton

`list_files_git` assumes that the package is at the root of the git repository. This breaks when trying to list files for a package in a subdirectory of a repo, or in other cases, e.g. if the user's home directory is a git repo.

diff --cc tests/test_cargo_compile_git_deps.rs
Simple merge