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

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.

No files, tests, or entry points are identified. Start by locating the code that creates insertions and the data used to identify a build and branch, then clarify whether an existing insertion should be updated or canceled. Done means duplicate same-build, same-branch insertions follow the agreed behavior with coverage for that case.

Written by the indexing model from the issue text.