Add option to only generate and upload migration archives

[jil-sc]

Description

Sometimes we want to rewrite the history during the migration, e.g. for LFS migration.

This page documents the approach: https://gist.github.com/joshjohanning/e3a08cb2ea877576f01f9188b20985c8

For this to be even easier, it would be useful to have an option to stop the migration once the migration archives has been created and uploaded, so that we could perform the required rewrites.