Fix bug in list_file_walk when cwd != path

Previously this was calling .is_file() on a relative path.  This would fail if
the path was not relative to the current working directory, for example when
fingerprinting a path dependency.
Test for build command fingerprints in path deps

This tests the fix for list_files_walk from #561.
auto merge of #561 : mbrubeck/cargo/list_file_walk, r=alexcrichton

Previously this was calling `.is_file()` on a relative path.  This would fail if the path was not relative to the current working directory, for example when listing files in a path dependency.