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.

Annex/CatFile.hs

@@ -1,6 +1,6 @@
 {- git cat-file interface, with handle automatically stored in the Annex monad
  -
- - Copyright 2011-2015 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2018 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -16,9 +16,11 @@ module Annex.CatFile (
 	catObjectMetaData,
 	catFileStop,
 	catKey,
+	catSymLinkTarget,
 	catKeyFile,
 	catKeyFileHEAD,
-	catSymLinkTarget,
+	catKeyFileHidden,
+	catObjectMetaDataHidden,
 ) where
 
 import qualified Data.ByteString.Lazy as L
@@ -34,6 +36,8 @@ import Git.FilePath
 import Git.Index
 import qualified Git.Ref
 import Annex.Link
+import Annex.CurrentBranch
+import Types.AdjustedBranch
 import Utility.FileSystemEncoding
 
 catFile :: Git.Branch -> FilePath -> Annex L.ByteString
@@ -142,3 +146,16 @@ catKeyFile f = ifM (Annex.getState Annex.daemon)
 
 catKeyFileHEAD :: FilePath -> Annex (Maybe Key)
 catKeyFileHEAD f = catKey $ Git.Ref.fileFromRef Git.Ref.headRef f
+
+{- Look in the original branch from whence an adjusted branch is based
+ - to find the file. But only when the adjustment hides some files. -}
+catKeyFileHidden :: FilePath -> CurrBranch -> Annex (Maybe Key) 
+catKeyFileHidden = hiddenCat catKey
+
+catObjectMetaDataHidden :: FilePath -> CurrBranch -> Annex (Maybe (Integer, ObjectType))
+catObjectMetaDataHidden = hiddenCat catObjectMetaData
+
+hiddenCat :: (Ref -> Annex (Maybe a)) -> FilePath -> CurrBranch -> Annex (Maybe a)
+hiddenCat a f (Just origbranch, Just adj)
+	| adjustmentHidesFiles adj = a (Git.Ref.fileFromRef origbranch f)
+hiddenCat _ _ _ = return Nothing