Push a tag sometimes does not trigger a pipeline

Context

Some customers have reported that pipelines are not triggered when pushing tags refs. In this case, tags are created but will not run pipelines.

Possible solutions

Call Gitaly `RefExists` RPC [outdated]

We should call Gitaly `RefExists` RPC when there is a tag ref not found in Redis cache. This will work as a fallback to validate the latest repository state.

Determine whether the issue has been resolved by Atomically refresh branch/tag cache in PostReceive (!160946 - merged).