Remove duplicate computation of the to_check set
Remove duplicate computation of the to_check set

Summary: This value was computed twice in `include_dependencies_and_dependents`. Now it's computed only once. It's also clearer to use the name `to_check` when calculating `preliminary_to_merge`

Reviewed By: mroch

Differential Revision: D19910655

fbshipit-source-id: a1a70709a4ae3e53bd1487d921e72dd8c3c8ccc3

mroch/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow