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/Multicast.hs

@@ -129,7 +129,9 @@ send ups fs = do
 	-- expensive.
 	starting "sending files" (ActionItemOther Nothing) $
 		withTmpFile "send" $ \t h -> do
-			fs' <- seekHelper LsFiles.inRepo =<< workTreeItems fs
+			let ww = WarnUnmatchLsFiles
+			fs' <- seekHelper ww LsFiles.inRepo
+				=<< workTreeItems ww fs
 			matcher <- Limit.getMatcher
 			let addlist f o = whenM (matcher $ MatchingFile $ FileInfo f f) $
 				liftIO $ hPutStrLn h o