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/Uninit.hs

@@ -41,11 +41,13 @@ check = do
 
 seek :: CmdParams -> CommandSeek
 seek ps = do
-	l <- workTreeItems ps
+	l <- workTreeItems ww ps
 	withFilesNotInGit (commandAction . whenAnnexed (startCheckIncomplete . fromRawFilePath)) l
 	Annex.changeState $ \s -> s { Annex.fast = True }
-	withFilesInGit (commandAction . whenAnnexed Command.Unannex.start) l
+	withFilesInGit ww (commandAction . whenAnnexed Command.Unannex.start) l
 	finish
+  where
+	ww = WarnUnmatchLsFiles
 
 {- git annex symlinks that are not checked into git could be left by an
  - interrupted add. -}