refs/migrate: Add a flag to ignore reflogs during migration

The git-refs(1) command's migrate action is used to migrate a repository from one reference backend to another. It also migrates reflogs alongside refs. Repositories on the server generally forego reflogs. So it would be nice to have a flag to ignore reflogs during migration, which would skip reflogs if present (this could happen due to misconfig on the server side or old repositories which contained reflogs but are no longer used/needed).