migrate: support adding size back to URL keys

migrate: support adding size back to URL keys

migrate: Support adding size to URL keys that were added with --relaxed, by
running eg: git-annex migrate --backend=URL foo

Since url keys cannot be generated, that used to fail. Make it notice that
the backend is not changed, and just get the size of the content.

Sponsored-by: Brock Spratlen on Patreon