sparse: don't crash in non-sparse repositories
Update the workingfilectxdata() to check that this is actually a sparse
repository before trying to call repo.sparsematch()
Test Plan: Update the test to confirm that "hg diff" works instead of crashes now.
Reviewers: #fbhgext, mbthomas
Reviewed By: #fbhgext, mbthomas
Differential Revision: https://phab.mercurial-scm.org/D1023