Fix list_files_git for paths in subdirs of a repo

list_files_git previously assumed that the package was always at the root of
the git repository.  This broke 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.
Test for build commands in git subdirs
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.