Fixed comment on extractNonDeletionChanges.
Fixed comment on extractNonDeletionChanges.

This commit fixes the comment for extractNonDeletionChanges, which
previously didn't account for the case of there being no non-deletion
changes.

Signed-off-by: Jacob Howard <f862f167b85d41b225785c70d70808bc7337c1fe@havoc.io>

havoc-io/mutagen,havoc-io/mutagen