record only subtree in export db and log after importing a subtree

Annex/Import.hs

@@ -107,9 +107,7 @@ buildImportCommit remote importtreeconfig importcommitconfig importable =
 		mkcommits origtree basecommit imported >>= \case
 			Nothing -> return Nothing
 			Just finalcommit -> do
-				updateexportdb finaltree
-				oldexport <- updateexportlog finaltree
-				updatelocationlog oldexport finaltree
+				updatestate finaltree
 				return (Just finalcommit)
 
 	mkcommits origtree basecommit (History importedtree hs) = do
@@ -127,6 +125,18 @@ buildImportCommit remote importtreeconfig importcommitconfig importable =
 					importedtree
 				return (Just commit)
 	
+	updatestate committedtree = do
+		importedtree <- case subdir of
+			Nothing -> pure committedtree
+			Just dir -> 
+				let subtreeref = Ref $
+					fromRef committedtree ++ ":" ++ getTopFilePath dir
+				in fromMaybe emptyTree
+					<$> inRepo (Git.Ref.tree subtreeref)
+		updateexportdb importedtree
+		oldexport <- updateexportlog importedtree
+		updatelocationlog oldexport importedtree
+
 	updateexportdb importedtree = do
 		db <- Export.openDb (Remote.uuid remote)
 		Export.writeLockDbWhile db $ do

doc/todo/import_tree.mdwn

@@ -10,14 +10,6 @@ this.
 
 ## implementation notes
 
-* When doing `git annex export master:subdir` followed by
-  `git annex import master:subdir` which imports a new file,
-  followed by merging the import and re-export, it does unncessary
-  work deleting stuff not in the subdir from the remote. 
-  
-  Why? Seems that the import recorded the exported tree wrong, using the
-  whole tree and not the sub-tree.
-
 * Does sync --content with remote.name.annex-tracking-branch=master:subdir
   export the right tree and update the remote tracking branch right?
   Does it import correctly?