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/Inprogress.hs

@@ -38,9 +38,11 @@ seek o = do
 			| otherwise -> commandAction stop
 		_ -> do
 			let s = S.fromList ts
-			withFilesInGit
+			withFilesInGit ww
 				(commandAction . (whenAnnexed (start s)))
-				=<< workTreeItems (inprogressFiles o)
+				=<< workTreeItems ww (inprogressFiles o)
+  where
+	ww = WarnUnmatchLsFiles
 
 start :: S.Set Key -> RawFilePath -> Key -> CommandStart
 start s _file k