graphmod: compute slow revset query once prior to reachableroots (issue4782) Because revsets query is evaluated lazily, "list(revs)" may take long for complicated query. So we shouldn't iterate revs many times. This patch is the easiest workaround for the issue4782. We could introduce more aggressive caching, but it wouldn't be as fast as the simple baseset operation. Gregory Szorc said "this makes `hg wip` on my Firefox clone ~4x faster than 3.5.1 (~6.5s to ~1.5s). This is after a regression in @ to ~45s."