obshistory: prepare data for displaymarkers() in a separate function

obshistory: make patchavailable() handle predecessors too

tests: make sure not to test the same thing twice A block under this one already tests obslog --no-graph with -f. Right now there's no difference in output, but maybe some day obslog --no-graph will actually support --filternonlocal.

obshistory: rename precursors to predecessors in displayrevs()