With the new release of Git, version 2.9.1, our "pull zero len file" test is
broken. @larsxschneider was the first to point this out in github/git-lfs#1363.
The cause of this problem is that `git-status` changed to print "working tree
clean", instead of "working directory clean" when running in a clean working
tree. Though this is more correct, it still broke our tests :-).
This commit changes the `grep` assertion to match the new behavior of
`git-status`.
The relevant change from git/git is here: git/git@2a0e6cd.
Closes: github/git-lfs#1363.
