avoid borg list of archives that have been listed before

This makes sync a lot faster in the common case where there's no new
backup.

There's still room for it to be faster. Currently the old imported tree
has to be traversed, to generate the ImportableContents. Which then
gets turned around to generate the new imported tree, which is
identical. So, it would be possible to just return a "no new imports",
or an ImportableContents that has a way to graft in a tree. The latter
is probably too far to go to optimise this, unless other things need it.
The former might be worth it, but it's already pretty fast, since git
ls-tree is pretty fast.

Git/LsTree.hs

@@ -1,17 +1,17 @@
 {- git ls-tree interface
  -
- - Copyright 2011-2019 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2020 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
-{-# LANGUAGE BangPatterns #-}
-
 module Git.LsTree (
 	TreeItem(..),
 	LsTreeMode(..),
 	lsTree,
 	lsTree',
+	lsTreeStrict,
+	lsTreeStrict',
 	lsTreeParams,
 	lsTreeFiles,
 	parseLsTree,
@@ -30,6 +30,7 @@ import Data.Either
 import System.Posix.Types
 import qualified Data.ByteString as S
 import qualified Data.ByteString.Lazy as L
+import qualified Data.Attoparsec.ByteString as AS
 import qualified Data.Attoparsec.ByteString.Lazy as A
 import qualified Data.Attoparsec.ByteString.Char8 as A8
 
@@ -38,7 +39,7 @@ data TreeItem = TreeItem
 	, typeobj :: S.ByteString
 	, sha :: Ref
 	, file :: TopFilePath
-	} deriving Show
+	} deriving (Show)
 
 data LsTreeMode = LsTreeRecursive | LsTreeNonRecursive
 
@@ -51,6 +52,13 @@ lsTree' ps lsmode t repo = do
 	(l, cleanup) <- pipeNullSplit (lsTreeParams lsmode t ps) repo
 	return (rights (map parseLsTree l), cleanup)
 
+lsTreeStrict :: LsTreeMode -> Ref -> Repo -> IO [TreeItem]
+lsTreeStrict = lsTreeStrict' []
+
+lsTreeStrict' :: [CommandParam] -> LsTreeMode -> Ref -> Repo -> IO [TreeItem]
+lsTreeStrict' ps lsmode t repo = rights . map parseLsTreeStrict
+	<$> pipeNullSplitStrict (lsTreeParams lsmode t ps) repo
+
 lsTreeParams :: LsTreeMode -> Ref -> [CommandParam] -> [CommandParam]
 lsTreeParams lsmode r ps =
 	[ Param "ls-tree"
@@ -83,6 +91,13 @@ parseLsTree b = case A.parse parserLsTree b of
 	A.Done _ r  -> Right r
 	A.Fail _ _ err -> Left err
 
+parseLsTreeStrict :: S.ByteString -> Either String TreeItem
+parseLsTreeStrict b = go (AS.parse parserLsTree b)
+  where
+	go (AS.Done _ r) = Right r
+	go (AS.Fail _ _ err) = Left err
+	go (AS.Partial c) = go (c mempty)
+
 {- Parses a line of ls-tree output, in format:
  - mode SP type SP sha TAB file
  -