revlog: adapt the `reading` check for `unionrepo`
We cannot just rely on the length check for the `unionrepo` as the local revlog
might be empty while the other revlog contains data. In addition, we need to
also open the second revlog for reading when needed.
#require cargo test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/../rust
Check if Cargo.lock is up-to-date. Will fail with a 101 error code if not.
$ cargo check --locked --all --quiet
However most CIs will run `cargo build` or similar before running the tests, so we need to check if it was modified
$ testrepohg diff Cargo.lock