Use Redis pipeline creations for MR commit push pipelines

The following discussion from !165363 (merged) should be addressed:

• @hfyngvason started a discussion: (+1 comment)

  Not blocking: Should this method also be used here? This is the code path used in PostReceive (i.e. for MR pipelines triggered by pushes).

  It could make the UI appear more responsive to pushes, and it could also slightly improve mergeability check races between branch pipelines, MR pipelines, and external pipelines.

  Edit: Not blocking because if a request isn't registered, it won't affect the mergeability check.