started migrate --update

This is most of the way there, but not quite working.

The layout of migrate.tree/ needs to be changed to follow this approach.
git log will list all the files in tree order, so the new layout needs
to alternate old and new keys. Can that be done? git may not document
tree order, or may not preserve it here.

Alternatively, change to using git log --format=raw and extract
the tree header from that, then use
git diff --raw $tree:migrate.tree/old $tree:migrate.tree/new
That will be a little more expensive, but only when there are lots of
migrations.

Sponsored-by: Joshua Antonishen on Patreon

Logs/Export.hs

@@ -78,7 +78,7 @@ recordExportBeginning remoteuuid newtree = do
 -- repository, the tree has to be kept available, even if it
 -- doesn't end up being merged into the master branch.
 recordExportTreeish :: Git.Ref -> Annex ()
-recordExportTreeish t = 
+recordExportTreeish t = void $
 	Annex.Branch.rememberTreeish t (asTopFilePath exportTreeGraftPoint)
 
 -- | Record that an export to a special remote is under way.