rust-changelog: don't panic on empty file lists

tests: use sha256line.py instead of /dev/random in test-censor.t (issue6858) Sometimes the systems that run our test suite don't have enough entropy and they cannot produce target file of the expected size using /dev/random, which results in test failures. Switching to /dev/urandom would give us way more available data at the cost of it being less "random", but we don't really need to use entropy for this task at all, since we only care if the file size after compression is big enough to not be stored inline in the revlog. So let's use something that we already have used to generate this kind of data in other tests.