Speedup revlog.ancestors for the linear case revlog.ancestors can be expensive on big repos. This cuts down the overall time for hg update by ~19% by short cutting revlog.ancestors when one of the revisions is reachable from another.

port:4:export
port:4:vaportight
port:4:import/export
FIXME: history is wrong here
port:1:1:-:eggs:import
port:1:2:+:eggs:vaportight
port:1:3:+:eggs:import/export
port:0:2:+:spam:export
port:0:1:+:spam:import
port:4:import/export