sync --content now supports --hide-missing adjusted branches

This relies on git ls-files --with-tree, which I'm using in a way that
its man page does not document. Hm. I emailed the git list to try to get
the docs improved, but at least the git test suite does test the same
kind of use case I'm using here.

Performance impact when not in an adjusted branch is limited to some
additional MVar accesses, and a single git call to determine the name of
the current branch. So very minimal.

When in an adjusted branch, the performance impact is
in Annex.WorkTree.lookupFile, which starts doing an equal amount of work
for files that didn't exist as it already did for files that were
unlocked.

This commit was sponsored by Jochen Bartl on Patreon.

CmdLine/Seek.hs

@@ -29,6 +29,7 @@ import Logs.Transfer
 import Remote.List
 import qualified Remote
 import Annex.CatFile
+import Annex.CurrentBranch
 import Annex.Content
 import Annex.InodeSentinal
 import qualified Database.Keys
@@ -270,17 +271,33 @@ seekHelper a l = inRepo $ \g ->
 -- An item in the work tree, which may be a file or a directory.
 newtype WorkTreeItem = WorkTreeItem FilePath
 
+-- When in an adjusted branch that hides some files, it may not exist
+-- in the current work tree, but in the original branch. This allows
+-- seeking for such files.
+newtype AllowHidden = AllowHidden Bool
+
 -- Many git commands seek work tree items matching some criteria,
 -- and silently skip over anything that does not exist. But users expect
 -- an error message when one of the files they provided as a command-line
 -- parameter doesn't exist, so this checks that each exists.
 workTreeItems :: CmdParams -> Annex [WorkTreeItem]
-workTreeItems ps = do
+workTreeItems = workTreeItems' (AllowHidden False)
+
+workTreeItems' :: AllowHidden -> CmdParams -> Annex [WorkTreeItem]
+workTreeItems' (AllowHidden allowhidden) ps = do
+	currbranch <- getCurrentBranch
 	forM_ ps $ \p ->
-		unlessM (isJust <$> liftIO (catchMaybeIO $ getSymbolicLinkStatus p)) $ do
+		unlessM (exists p <||> hidden currbranch p) $ do
 			toplevelWarning False (p ++ " not found")
 			Annex.incError
 	return (map WorkTreeItem ps)
+  where
+	exists p = isJust <$> liftIO (catchMaybeIO $ getSymbolicLinkStatus p)
+	hidden currbranch p
+		| allowhidden = do
+			f <- liftIO $ relPathCwdToFile p
+			isJust <$> catObjectMetaDataHidden f currbranch
+		| otherwise = return False
 
 notSymlink :: FilePath -> IO Bool
 notSymlink f = liftIO $ not . isSymbolicLink <$> getSymbolicLinkStatus f