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/View.hs

@@ -101,7 +101,7 @@ checkoutViewBranch view mkbranch = do
 		 - removed.) -}
 		top <- liftIO . absPath . fromRawFilePath =<< fromRepo Git.repoPath
 		(l, cleanup) <- inRepo $
-			LsFiles.notInRepoIncludingEmptyDirectories False
+			LsFiles.notInRepoIncludingEmptyDirectories [] False
 				[toRawFilePath top]
 		forM_ l (removeemptydir top)
 		liftIO $ void cleanup