optimisation: use graftTree to remember the export branch

Sped up git-annex export in repositories with lots of keys.

Old method read whole git-annex branch tree into memory.

Annex/Branch.hs

@@ -24,7 +24,7 @@ module Annex.Branch (
 	forceCommit,
 	getBranch,
 	files,
-	graftTreeish,
+	rememberTreeish,
 	performTransitions,
 	withIndex,
 ) where
@@ -646,17 +646,15 @@ getMergedRefs' = do
  - and then removes it. This ensures that the treeish won't get garbage
  - collected, and will always be available as long as the git-annex branch
  - is available. -}
-graftTreeish :: Git.Ref -> TopFilePath -> Annex ()
-graftTreeish treeish graftpoint = lockJournal $ \jl -> do
+rememberTreeish :: Git.Ref -> TopFilePath -> Annex ()
+rememberTreeish treeish graftpoint = lockJournal $ \jl -> do
 	branchref <- getBranch
 	updateIndex jl branchref
-	Git.Tree.Tree t <- inRepo $
-		Git.Tree.getTree Git.LsTree.LsTreeRecursive branchref
-	t' <- inRepo $ Git.Tree.recordTree $ Git.Tree.Tree $
-		Git.Tree.RecordedSubTree graftpoint treeish [] : t
+	origtree <- fromMaybe (giveup "unable to determine git-annex branch tree") <$>
+		inRepo (Git.Ref.tree branchref)
+	addedt <- inRepo $ Git.Tree.graftTree treeish graftpoint origtree
 	c <- inRepo $ Git.Branch.commitTree Git.Branch.AutomaticCommit
-		"graft" [branchref] t'
-	origtree <- inRepo $ Git.Tree.recordTree (Git.Tree.Tree t)
+		"graft" [branchref] addedt
 	c' <- inRepo $ Git.Branch.commitTree Git.Branch.AutomaticCommit
 		"graft cleanup" [c] origtree
 	inRepo $ Git.Branch.update' fullname c'

CHANGELOG

@@ -2,6 +2,7 @@ git-annex (7.20190220) UNRELEASED; urgency=medium
 
   * Fix storage of metadata values containing newlines.
     (Reversion introduced in version 7.20190122.)
+  * Sped up git-annex export in repositories with lots of keys.
 
  -- Joey Hess <id@joeyh.name>  Wed, 20 Feb 2019 14:20:59 -0400
 

Logs/Export.hs

@@ -124,7 +124,7 @@ recordExportBeginning remoteuuid newtree = do
 		buildExportLog 
 			. changeMapLog c ep new
 			. parseExportLog
-	Annex.Branch.graftTreeish newtree (asTopFilePath "export.tree")
+	Annex.Branch.rememberTreeish newtree (asTopFilePath "export.tree")
 
 parseExportLog :: L.ByteString -> MapLog ExportParticipants Exported
 parseExportLog = parseMapLog exportParticipantsParser exportedParser