verify: fix "missing revlog!" errors for revlog format v0 and add test
With revlog format v0 the .d files are empty if the only revision stored is an
empty file. Since Mercurial can no longer create format v0 repositories, but
still use it, add a script which creates a repository with a single empty file.
This can be used in other tests if wanted.
(binary:application/octet-stream)