unionrevlog: extract 'baserevision' and 'baserevdiff' methods
This makes possible to use unionrevlog class with subclasses of revlog that
override revlog's 'revision' and 'revdiff' methods. In particular this change
is necessary to implement manifest compression, as it allows extension to
replace manifest class and override 'revision' amd 'revdiff' methods there.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.