censors: simply use `revlog.reading` to keep things open dring rewrite We have a dedicated context now, we can simply use it. In practice, we cannot "simply" use it, as `finddeltainfo` still requires the file pointer to be passed explicitly. We will keep the old context manager open for a small bit until we get rid of it in a future changeset.

stream-clone: use `revlog.reading` in `revlog.get_streams` We can get the same result while using more standard method, so we do.

debug-revlog: keep the revlog open for the analysis duration This is more explicit. No performance differences were observed.

repo-upgrade: keep the revlog open while cloning This is more explicit. No performance differences were observed.

verify: keep the revlog open for reading while verifying it This is more explicit. No performances difference were observed.

convert: use `changelogrevision` to fetch changeset data instead of read This `changelogrevision` is a higher level function that will remain simple to use as we refactor the revlog.

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.

revlog: adapt the `reading` check for `bundlerepo` We cannot just rely on the length check for the `bundlerepo` as the local revlog might be empty with all data in the bundle.