migrate: New --remove-size option

migrate: New --remove-size option

While intended for converting URL keys added by addurl --fast to be
as if added by addurl --relaxed, it can also be used to remove size
from other types of keys. Although that is not likely to be useful
for checksummed keys, I suppose it could be used for WORM or other
non-checksum keys.

Specifying the --remove-size option does not prevent other migrations
from taking effect if there's a key upgrade to perform, or if the
backend has changed. So --backend=URL needs to be used to prevent
migrating an URL key to the default backend.

Note that it's not possible to use git-annex migrate to convert from a
non-URL key to an URL key, as URL keys cannot be generated, except by
addurl. So while this can get the same effect as --relaxed would have
when addurl --fast was used, when --fast was not used, it won't work, or
if --backend=URL is not used will remove the size but not prevent
checksum verification, which is not useful. Due to this complexity, I
decided not to mention it in the git-annex addurl man page.

Sponsored-by: Jochen Bartl on Patreon