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

Annex/Branch.hs

@@ -14,6 +14,7 @@ module Annex.Branch (
 	hasSibling,
 	siblingBranches,
 	create,
+	getBranch,
 	UpdateMade(..),
 	update,
 	forceUpdate,
@@ -120,7 +121,7 @@ siblingBranches = inRepo $ Git.Ref.matchingUniq [name]
 create :: Annex ()
 create = void getBranch
 
-{- Returns the ref of the branch, creating it first if necessary. -}
+{- Returns the sha of the branch, creating it first if necessary. -}
 getBranch :: Annex Git.Ref
 getBranch = maybe (hasOrigin >>= go >>= use) return =<< branchsha
   where
@@ -920,10 +921,14 @@ getMergedRefs' = do
 {- Grafts a treeish into the branch at the specified location,
  - 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. -}
-rememberTreeish :: Git.Ref -> TopFilePath -> Annex ()
-rememberTreeish treeish graftpoint = lockJournal $ rememberTreeishLocked treeish graftpoint
-rememberTreeishLocked :: Git.Ref -> TopFilePath -> JournalLocked -> Annex ()
+ - is available.
+ -
+ - Returns the sha of the git commit made to the git-annex branch.
+ -}
+rememberTreeish :: Git.Ref -> TopFilePath -> Annex Git.Sha
+rememberTreeish treeish graftpoint = lockJournal $
+	rememberTreeishLocked treeish graftpoint
+rememberTreeishLocked :: Git.Ref -> TopFilePath -> JournalLocked -> Annex Git.Sha
 rememberTreeishLocked treeish graftpoint jl = do
 	branchref <- getBranch
 	updateIndex jl branchref
@@ -940,6 +945,7 @@ rememberTreeishLocked treeish graftpoint jl = do
 	-- and the index was updated to that above, so it's safe to
 	-- say that the index contains c'.
 	setIndexSha c'
+	return c'
 
 {- Runs an action on the content of selected files from the branch.
  - This is much faster than reading the content of each file in turn,