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/Fix.hs

@@ -32,9 +32,11 @@ cmd = noCommit $ withGlobalOptions [annexedMatchingOptions] $
 
 seek :: CmdParams -> CommandSeek
 seek ps = unlessM crippledFileSystem $ do 
-	withFilesInGit
+	withFilesInGit ww
 		(commandAction . (whenAnnexed $ start FixAll))
-		=<< workTreeItems ps
+		=<< workTreeItems ww ps
+  where
+	ww = WarnUnmatchLsFiles
 
 data FixWhat = FixSymlinks | FixAll