revbranchcache: return uncached branchinfo for nullrev (issue4683) This fixes the crash caused by "branch(null)" revset. No cache should be necessary for nullrev because changelog.branchinfo(nullrev) does not involve IO operation. Note that the problem of "branch(wdir())" isn't addressed by this patch. "wdir()" will raise TypeError in many places because of None. This is the reason why "wdir()" is still experimental.