Martin von Zweigbergk <martinvonz@google.com> [Wed, 23 Jan 2019 12:11:36 -0800] rev 4377
evolve: document the "if not shouldupdate" block
I missed the "not" in "if not shouldupdate: ... hg.updaterepo()" and
it took a long time for me to realize it's about updating *back* to
the original commit (or to its successor). Let's add a comment to
maybe help the person not make the same mistake.