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/Types.hs

@@ -135,7 +135,12 @@ fmtObjectType CommitObject = "commit"
 fmtObjectType TreeObject = "tree"
 
 {- Types of items in a tree. -}
-data TreeItemType = TreeFile | TreeExecutable | TreeSymlink | TreeSubmodule
+data TreeItemType
+	= TreeFile
+	| TreeExecutable
+	| TreeSymlink
+	| TreeSubmodule
+	| TreeSubtree
 	deriving (Eq, Show)
 
 {- Git uses magic numbers to denote the type of a tree item. -}
@@ -144,6 +149,7 @@ readTreeItemType "100644" = Just TreeFile
 readTreeItemType "100755" = Just TreeExecutable
 readTreeItemType "120000" = Just TreeSymlink
 readTreeItemType "160000" = Just TreeSubmodule
+readTreeItemType "040000" = Just TreeSubtree
 readTreeItemType _ = Nothing
 
 fmtTreeItemType :: TreeItemType -> S.ByteString
@@ -151,12 +157,14 @@ fmtTreeItemType TreeFile = "100644"
 fmtTreeItemType TreeExecutable = "100755"
 fmtTreeItemType TreeSymlink = "120000"
 fmtTreeItemType TreeSubmodule = "160000"
+fmtTreeItemType TreeSubtree = "040000"
 
 toTreeItemType :: FileMode -> Maybe TreeItemType
 toTreeItemType 0o100644 = Just TreeFile
 toTreeItemType 0o100755 = Just TreeExecutable
 toTreeItemType 0o120000 = Just TreeSymlink
 toTreeItemType 0o160000 = Just TreeSubmodule
+toTreeItemType 0o040000 = Just TreeSubtree
 toTreeItemType _ = Nothing
 
 fromTreeItemType :: TreeItemType -> FileMode
@@ -164,6 +172,7 @@ fromTreeItemType TreeFile = 0o100644
 fromTreeItemType TreeExecutable = 0o100755
 fromTreeItemType TreeSymlink = 0o120000
 fromTreeItemType TreeSubmodule = 0o160000
+fromTreeItemType TreeSubtree = 0o040000
 
 data Commit = Commit
 	{ commitTree :: Sha