revisionbranchcache: fall back to slow path if starting readonly (issue4531) Transitioning to Mercurial versions with revision branch cache could be slow as long as all operations were readonly (revset queries) and the cache would be populated but not written back. Instead, fall back to using the consistently slow path when readonly and the cache doesn't exist yet. That avoids the overhead of populating the cache without writing it back. If not readonly, it will still populate all missing entries initially. That avoids repeated writing of the cache file with small updates, and it also makes sure a fully populated cache available for the readonly operations.

lib	- sys sys
	mercurial	- sys sys
		hgrc.d	- sys sys
			9diff.rc	- sys sys
			factotum.rc	- sys sys
rc	- sys sys
	bin	- sys sys
		9diff	- sys sys
		hg	- sys sys
sys	- sys sys
	lib	- sys sys
		python	- sys sys
			lib	- sys sys
				python2.5	- sys sys
					site-packages	- sys sys
						hgext	- sys sys
							+	- sys sys
						mercurial	- sys sys
							+	- sys sys
						mercurial-VERSION-py2.5.egg-info	- sys sys
	src	- sys sys
		cmd	- sys sys
			hg	- sys sys
				+	- sys sys