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.
{"site_id":"cf023865-0eb1-49e9-951b-29555949abf0","path":"html/"}