Display a warning message when asked to operate on a file inside a directory that's a symbolic link to elsewhere

This relicates git's behavior. It adds a few stat calls for the command
line parameters, so there is some minor slowdown, but even with thousands
of parameters it will not be very noticable, and git does the same statting
in similar circumstances.

Note that this does not prevent eg "git annex add symlink"; the symlink
will be added to git as usual. And "git annex find symlink" will silently
list nothing as well. It's only "symlink/foo" or "subdir/symlink/foo" that
triggers the warning.

CmdLine/Seek.hs

@@ -243,28 +243,47 @@ newtype WorkTreeItem = WorkTreeItem FilePath
 -- seeking for such files.
 newtype AllowHidden = AllowHidden Bool
 
--- Many git commands seek work tree items matching some criteria,
+-- Many git commands like ls-files 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.
+--
+-- Also, when two directories are symlinked, referring to a file
+-- inside the symlinked directory will be silently skipped by git commands
+-- like ls-files. But, the user would be surprised for it to be skipped, so
+-- check if the parent directories are symlinks.
 workTreeItems :: CmdParams -> Annex [WorkTreeItem]
 workTreeItems = workTreeItems' (AllowHidden False)
 
 workTreeItems' :: AllowHidden -> CmdParams -> Annex [WorkTreeItem]
 workTreeItems' (AllowHidden allowhidden) ps = do
 	currbranch <- getCurrentBranch
-	forM_ ps $ \p ->
-		unlessM (exists p <||> hidden currbranch p) $ do
-			toplevelWarning False (p ++ " not found")
-			Annex.incError
+	forM_ ps $ \p -> do
+		relf <- liftIO $ relPathCwdToFile p
+		ifM (not <$> (exists p <||> hidden currbranch relf))
+			( prob (p ++ " not found")
+			, whenM (viasymlink (upFrom relf)) $
+				prob (p ++ " is beyond a symbolic link")
+			)
 	return (map (WorkTreeItem) ps)
   where
 	exists p = isJust <$> liftIO (catchMaybeIO $ getSymbolicLinkStatus p)
-	hidden currbranch p
-		| allowhidden = do
-			f <- liftIO $ relPathCwdToFile p
-			isJust <$> catObjectMetaDataHidden (toRawFilePath f) currbranch
+
+	viasymlink Nothing = return False
+	viasymlink (Just p) =
+		ifM (liftIO $ isSymbolicLink <$> getSymbolicLinkStatus p)
+			( return True
+			, viasymlink (upFrom p)
+			)
+
+	hidden currbranch f
+		| allowhidden = isJust
+			<$> catObjectMetaDataHidden (toRawFilePath f) currbranch
 		| otherwise = return False
 
+	prob msg = do
+		toplevelWarning False msg
+		Annex.incError
+	
 notSymlink :: RawFilePath -> IO Bool
 notSymlink f = liftIO $ not . isSymbolicLink <$> R.getSymbolicLinkStatus f