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unused log was out of date, eg a file used to be unused but then got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

doc/todo/dead_files_in_checkout_directly.mdwn

@@ -1,3 +1,5 @@
 When you want to dead a file in your checkout, you can only do so via the key of the file. You can find the corresponding key with a bit of bash like this: `git annex dead --key $(basename $(readlink file))` but that shouldn't be necessary IMO.
 
 It'd be a lot better if you could just dead files like this: `git annex dead --file file` or even like this: `git annex dead --file file1 file2 file3 otherfiles.*` (or maybe even like this: `git annex dead --file file1 file2 --key $key1 $key2`).
+
+> [[done]] in another way --[[Joey]]

doc/todo/dead_files_in_checkout_directly/comment_2_d70c820666b291c532f7814ca543a77a._comment

@@ -0,0 +1,26 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 2"""
+ date="2021-06-25T18:22:04Z"
+ content="""
+In <https://git-annex.branchable.com/forum/Forget_about_accidentally_added_file__63__/>
+there is an idea of `git annex unannex --forget file`
+
+And using unannex for this makes some sense; it's intended to be used to undo an
+accidental `git-annex add`. When it's used that way, and later `git-annex
+unused` finds the object file is not used by anything and the object gets
+deleted, fsck --all will start complaining about it.
+
+But there are still many ways it could go wrong. Being run recursively by
+accident. Or another file, perhaps in another branch, using the same key,
+which gets marked as dead.
+
+Hmm, `git annex dropunused` (or `drop --unused`)
+could mark the key as dead. At that point it's known to be unused.
+
+This way, the existing workflow of git-annex unannex followed by git-annex
+unused followed by dropping can be followed, and fsck --all does
+not later complain about the key.
+
+Done!
+"""]]