t7300: assert exact states of repo

Some of the tests in t7300 verify that git-clean(1) doesn't touch repositories that are embedded into the main repository. This is done by asserting a small set of substructures that are assumed to always exist, like the "refs/", "objects/" or "HEAD". This has the downside that we need to assume a specific repository structure that may be subject to change when new backends for the refdb land. At the same time, we don't thoroughly assert that git-clean(1) really didn't end up cleaning any files in the repository either. Convert the tests to instead assert that all files continue to exist after git-clean(1) by comparing a file listing via find(1) before and after executing clean. This makes our actual assertions stricter while having to care less about the repository's actual on-disk format. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>