obshistory: use a more appropriate type for obslog entries cand is actually a regular changeset and not 'M' (graphmod.MISSINGPARENT). And 'M' is only used for graph edges, but cand is drawn as a graph node. Visually there's no effect, but the code makes more sense this way.

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