The Gerrit review system tags every change (i.e. commit) with a durable Change-ID, which is represented int the log message, usually as the last line, like this:

  Change-Id: I7321a7fddab667af7a418eeeb7d48c0240570260

The Change-Id will be the same whenever that commit is cherry-picked anywhere, even if it has to change due to merging. It is not a hash, but a random UUID.

If you trowel through the log messages you can find the places where that change has "been" by looking for the Id.