perftweaks: micro optimization about branchcache.update
This patch did two micro optimizations:
- Avoid sorting headrevs since it's already sorted.
- Inline cl.node so there is no node hash table lookups inside the loop.
These are good practices. Although practically we don't have that many
headrevs to notice a difference.
Differential Revision: https://phab.mercurial-scm.org/D1452