dropping unused marks as dead

dropping unused marks as dead

Dropping an object with drop --unused or dropunused will mark it as
dead, preventing fsck --all from complaining about it after it's been
dropped from all repositories.

If another repository still has a copy, it won't be treated as dead
until it's also dropped from there.

The drop has to use --unused, can't be --key or something else, because
this indicates that the user has recently ran git-annex unused. If it
checked the unused log on every drop, bad things would happen when the
unused log was out of date, eg a file used to be unused but then got
re-added. Marking such a file as dead could be confusing. When the user
uses --unused/dropunused, they must consider the unused information to be
up-to-date.

The particular workflow this enables is:

	git annex add foo
	git annex unannex foo
	git annex unused
	git annex drop --unused / dropunused
	git annex fsck --all # no warnings

The docs for git-annex unannex say to use git-annex unused and dropunused,
so the user should be pointed in this direction when they want to undo an
accidental add.

Sponsored-by: Brock Spratlen on Patreon