evolve: make sure we always pick min revision to solve content-divergence

In current logic, to solve content-divergence between two csets
we pick one with minimum rev number as "divergent" and treat it as
local side (which is fixed); and other cset as "other" which may or may
not relocate.
For the information, later after this selection "divergent" and "other"
may or may not be swapped depending on the situation.

That said, this patch covers a missed case where it could select
a revision with maximum revision because it was just picking the
wdir parent.
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