manifest/revlog: do not let the revlog cache mutable objects If a buffer of an mutable object is passed to revlog.addrevision(), the revlog will happily store it in its cache. Later when the revlog reuses the cached entry, if the manifest modified the object in-between, all kind of bugs appears. We fix it by: - passing immutable objects to addrevision() if they are already available - only storing the text in the cache if it's of str type Then we can remove the conversion of the cache entry to str() during retrieval. That was probably just there hiding the bug for the common cases but not really fixing it.