rust-revlog: fix RevlogEntry.data() for NULL_REVISION Before this change, the pseudo-entry returned by `Revlog.get_entry` for `NULL_REVISION` would trigger errors in application code using it. For example, this fixes a crash spotted with changelog data while implementing RHGitaly: `Changelog.data_for_rev(-1)` was already returning the pseudo content as expected, e.g., for `hg log`, but `Changelog.entry_for_rev(-1).data()` would still crash with "corrupted revlog, hash check failed for revision -1". There is an added test for this scenario.