evolve: fix content-divergence resolution when p1 is null (issue6201) Before this fix, in cases like in the added test, the revision number -1 of the parent was misinterpreted to mean the tipmost revision instead of the null revision, causing the content-divergence resolution to fail.

test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t