As a user, I want at-mentions in commits to be suppressed when a repo's commits are computed for the first time. (BB-11097)

When pushing in a repo that has @mentions, the @mentions are triggered for each commit as the site walks the commit tree. I would like to suppress these as they have no context in a brand new repository.

When @mentions are triggered this way, it introduces a much higher chance of misfires - that is, notifications that are too old to be valid or are sent to the wrong user.