info: Added --dead-repositories option

I considered a more wide-ranging config option to make other commands
also show dead repositories. But it would be difficult to implement that
because Remote.keyLocations is used to get locations, filtering out dead
repos, and commands like get then try to use those locations. So a config
setting would make dead repos sometimes be acted on by commands.

Sponsored-by: unqueued on Patreon

doc/git-annex-dead.mdwn

@@ -12,7 +12,7 @@ This command exists to deal with situations where data has been lost,
 and you know it has, and you want to stop being reminded of that fact.
 
 When a repository is specified, indicates that the repository has
-been irretrievably lost, so it will not be listed in eg, `git annex info`.
+been irretrievably lost, so it will not be listed in eg, `git annex whereis`.
 Repositories can be specified using their remote name, their
 description, or their UUID. (To undo, use `git-annex semitrust`.)
 

doc/git-annex-info.mdwn

@@ -50,6 +50,11 @@ for the local repository and all annexed content.
   Display a list of special remotes that have been configured to
   autoenable.  
 
+* `--dead-repositories`
+
+  Display a list of repositories that have been marked as dead.
+  Such repositories are not displayed in other info displays.
+
 * matching options
 
   The [[git-annex-matching-options]](1) can be used to select what

doc/todo/show_dead_in_info___40__behind_option__63____41__.mdwn

@@ -1 +1,3 @@
 Would it be possible for `git annex info` to also show dead remotes (behind an option is fine), to make it easy to find what dead remotes there are? I needed to check what I had done for a service that was going away (AARNET Cloudstor, which used webdav), and was confused as to why it wasn't appearing anywhere.
+
+> Implemented `git-annex info --dead-repositories` [[done]] --[[Joey]]