refactor flow_ast_differ to use a more monoidal approach to diff joining
refactor flow_ast_differ to use a more monoidal approach to diff joining

Summary: move Option.(all diffs >>| List.concat) pattern out into helper function to make it clearer exactly what is happening when joining two diffs together.

Reviewed By: nmote

Differential Revision: D10414824

fbshipit-source-id: c2e5f323f54dafd55ebbb9b748c1b13cacce4fae

popham/flow,facebook/flow,gabelevi/flow,gabelevi/flow,popham/flow,facebook/flow,facebook/flow,samwgoldman/flow,samwgoldman/flow,jamesgpearce/flow,gabelevi/flow,popham/flow,mroch/flow,jamesgpearce/flow,gabelevi/flow,popham/flow,AgentME/flow,mroch/flow,jamesgpearce/flow,popham/flow,gabelevi/flow,AgentME/flow,jamesgpearce/flow,mroch/flow,popham/flow,AgentME/flow,samwgoldman/flow,jamesgpearce/flow,jamesgpearce/flow,popham/flow,gabelevi/flow,samwgoldman/flow,AgentME/flow,jamesgpearce/flow,samwgoldman/flow,samwgoldman/flow,mroch/flow,facebook/flow,gabelevi/flow,facebook/flow,mroch/flow,facebook/flow,samwgoldman/flow,mroch/flow,facebook/flow,mroch/flow,AgentME/flow