distributed migration for pull and sync --content

pull, sync: When operating on content, automatically hard link objects
that have been migrated.

Added annex.syncmigrations config that can be set to false to prevent
pull and sync from migrating object content.

I think that true is a good default for this config, because it avoids
users having to re-download migrated content or learning about migration.
But, some users will surely not like it, whether because it does take some
time (especially for the first git-annex branch scan when there is a long
history), or because they want to deal with it manually, or because their
filesystem doesn't support hard links and they don't want it to copy
objects.

Sponsored-by: k0ld on Patreon