Made several special remotes support locking content on them while
dropping, which allows dropping from another special remote when the
content will only remain on a special remote of these types.
In both cases, verify the content is present actively, because it's
certianly possible for things other than git-annex to have removed it.
Worth thinking about what to do if at some later point, git-lfs gains
support for dropping content, and a content locking operation.
That would probably need a transition; first would need to make lockContent
use the locking operation. Then, once enough time had passed that we can
assume any git-annex operating on the git-lfs remote had that change,
git-annex could finally allow dropping from git-lfs.
Or, it could be that git-lfs gains support for dropping content, but not
locking it. In that case, it seems this commit would need to be reverted,
and then wait long enough for that git-annex to be everywhere, and only
then can git-annex safely support dropping from git-lfs.
So, the assumption made in this commit could lead to bother later.. But I
think it's actually highly unlikely git-lfs does ever support dropping;
it's outside their centralized model. Probably. :) Worth keeping in mind as
the same assumption is made about other special remotes though.
This commit was sponsored by Ethan Aubin.
3175015d1b