Avoid grafting in export tree objects that are missing

They could be missing due to an interrupted git-annex at just the wrong
time during a prior graft, after which the tree objects got garbage
collected.

Or they could be missing because of manual messing with the git-annex
branch, eg resetting it to back before the graft commit.

Sponsored-by: Dartmouth College's OpenNeuro project

Annex/Branch.hs

@@ -889,9 +889,13 @@ performTransitionsLocked jl ts neednewlocalbranch transitionedrefs = do
 		return c
 	  where
 		regraft [] c = pure c
-		regraft (et:ets) c = 
-			prepRememberTreeish et graftpoint c
-				>>= regraft ets
+		regraft (et:ets) c =
+			-- Verify that the tree object exists.
+			catObjectDetails et >>= \case
+				Just _ ->
+					prepRememberTreeish et graftpoint c
+						>>= regraft ets
+				Nothing -> regraft ets c
 		graftpoint = asTopFilePath exportTreeGraftPoint
 
 checkBranchDifferences :: Git.Ref -> Annex ()