Martin von Zweigbergk <martinvonz@google.com> [Fri, 19 Apr 2019 09:41:39 -0700] rev 4607
evolve: run in a single transaction
This speeds up `hg evolve --all` of a stack of 24 commits in the hg
repo from 11.5s to 3.7s (3x). As can be seen in the tests, it also
avoid some warnings about transiently unstable commits.