sparse: treat paths as cwd-relative This commit makes it so sparse treats passed paths as CWD-relative, not repo-root-realive. This is a more intuitive behavior in my (and some other FB people's) opinion. This is breaking change however. My hope here is that since sparse is experimental, it's ok to introduce BCs. The reason (glob)s are needed in the test is this: in these two cases we do not supply path together with slashes, but `os.path.join` adds them, which means that under Windows they can be backslashes. To demonstrate this behavior, one could remove the (glob)s and run `./run-tests.py test-sparse.t` from MinGW's terminal on Windows.