Add switch to cancel/update insertions of same build to same branch

[RikkiGibson]

If, for some reason, we're creating multiple insertions of the same build to the same branch, we'd like to minimize clutter and resource usage by ensuring that we either cancel or update the existing insertion.

Intuitively it seems better to just update the existing insertion, in case any of the data in it is useful, but canceling the existing insertion might also be fine.