unionrepo: fix mismatches with revlog classes This is a subset of cfd30df0f8e4, applied to `unionrepository`. There are none of the `write()` method overrides here, like `bundlerepository`. With these changes, pytype flags the `unionrevlog` constructor: File "/mnt/c/Users/Matt/hg/mercurial/unionrepo.py", line 55, in __init__: No attribute '_revlog' on mercurial.changelog.changelog [attribute-error] Called from (traceback): line 207, in __init__ File "/mnt/c/Users/Matt/hg/mercurial/unionrepo.py", line 55, in __init__: No attribute '_revlog' on mercurial.revlog.revlog [attribute-error] Called from (traceback): line 232, in __init__ But it turns out that both `changelog.changelog` and `revlog.revlog` do have a `target` attribute, so they wouldn't trip over this. It seems weird that the second caller to be flagged is passing the private `_revlog`, but maybe that's how it needs to be.