Move current mirroring logic to its own method
Move current mirroring logic to its own method

We are going to have custom Starlark mirroring actions but
we don't want to migrate our users for now. Separating the
current logic in this change before adding Starlark actions.

BUG=185208243

Change-Id: Ifaefae42e5501fc20a0fcde6131d41cc54114d08

google/copybara,google/copybara,google/copybara