branchmap: stop useless rev -> node -> rev round trip We never use the node of new revisions unless in the very specific case of closed heads. So we can just use the revision number. So give another handfull of percent speedup.

branchmap: stop membership test in update logic Now that no user try to update the cache on a truncated repo we can drop the extra lookup. Give an handfull percent speedup on big branchmap update.

branchmap: remove silly line break The line fit in 80 character limit without it. It is even shorter without it.