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/Add.hs

@@ -82,10 +82,15 @@ seek o = startConcurrency commandStages $ do
 				giveup "--update --batch is not supported"
 			| otherwise -> batchFilesMatching fmt (gofile . toRawFilePath)
 		NoBatch -> do
-			l <- workTreeItems (addThese o)
-			let go a = a (commandAction . gofile) l
+			-- Avoid git ls-files complaining about files that
+			-- are not known to git yet, since this will add
+			-- them. Instead, have workTreeItems warn about other
+			-- problems, like files that don't exist.
+			let ww = WarnUnmatchWorkTreeItems
+			l <- workTreeItems ww (addThese o)
+			let go a = a ww (commandAction . gofile) l
 			unless (updateOnly o) $
-				go withFilesNotInGit
+				go (const withFilesNotInGit)
 			go withFilesMaybeModified
 			go withUnmodifiedUnlockedPointers