--all does not skip dead keys for all commands;
git-annex fsck checks if a key is dead and skips it.
When a key is specified, indicates that the content of that key has been
irretrievably lost. This prevents commands like `git annex fsck --all`
-from complaining about it; `--all` will not operate on the key anymore.
+from complaining about it.
(To undo, add the key's content back to the repository,
by using eg, `git-annex reinject`.)