annex.skipunknown with transition plan

Added annex.skipunknown git config, that can be set to false to change the
behavior of commands like `git annex get foo*`, to not skip over files/dirs
that are not checked into git and are explicitly listed in the command
line.

Significant complexity was needed to handle git-annex add, which uses some
git ls-files calls, but needs to not use --error-unmatch because of course
the files are not known to git.

annex.skipunknown is planned to change to default to false in a
git-annex release in early 2022. There's a todo for that.

Command/Copy.hs

@@ -51,8 +51,10 @@ seek o = startConcurrency commandStages $ do
 		NoBatch -> withKeyOptions
 			(keyOptions o) (autoMode o)
 			(commandAction . Command.Move.startKey (fromToOptions o) Command.Move.RemoveNever)
-			(withFilesInGit $ commandAction . go)
-			=<< workTreeItems (copyFiles o)
+			(withFilesInGit ww $ commandAction . go)
+			=<< workTreeItems ww (copyFiles o)
+  where
+	ww = WarnUnmatchLsFiles
 
 {- A copy is just a move that does not delete the source file.
  - However, auto mode avoids unnecessary copies, and avoids getting or