blackbox: fix blackbox causing exceptions in tests
In the tests some scripts call reposetup with the base ui instead of the
one the extensions have modified. This causes an exception in
blackbox.reposetup since it expected a method to be there. So I just
check for it first. This only happened when the blackbox extension
was enabled during tests.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.