migrate --update gets keys

The git log is outputting the diff, but this only looks at the new
files. When we have a new file, we can get the old filename by just
replacing "new" with "old". And then use branchFileRef to refer to it
allows catting the old key.

While this does have to skip past the old files in the diff, it's still
faster than calling git diff separately.

Sponsored-by: Nicholas Golder-Manning on Patreon

Command/Log.hs

@@ -165,7 +165,7 @@ startAll o outputter = do
 	void $ liftIO cleanup
 	stop
 
-{- Displays changes made. Only works when all the RefChanges are for the
+{- Displays changes made. Only works when all the LoggedFileChanges are for the
  - same key. The method is to compare each value with the value
  - after it in the list, which is the old version of the value.
  -
@@ -179,7 +179,7 @@ startAll o outputter = do
  - This also generates subtly better output when the git-annex branch
  - got diverged.
  -}
-showLogIncremental :: Outputter -> [RefChange Key] -> Annex ()
+showLogIncremental :: Outputter -> [LoggedFileChange Key] -> Annex ()
 showLogIncremental outputter ps = do
 	sets <- mapM (getset newref) ps
 	previous <- maybe (return genesis) (getset oldref) (lastMaybe ps)
@@ -196,7 +196,7 @@ showLogIncremental outputter ps = do
 {- Displays changes made. Streams, and can display changes affecting
  - different keys, but does twice as much reading of logged values
  - as showLogIncremental. -}
-showLog :: (ActionItem -> Outputter) -> [RefChange Key] -> Annex ()
+showLog :: (ActionItem -> Outputter) -> [LoggedFileChange Key] -> Annex ()
 showLog outputter cs = forM_ cs $ \c -> do
 	let ai = mkActionItem (changed c)
 	new <- S.fromList <$> loggedLocationsRef (newref c)
@@ -279,7 +279,7 @@ compareChanges format changes = concatMap diff changes
  - once the location log file is gone avoids it checking all the way back
  - to commit 0 to see if it used to exist, so generally speeds things up a
  - *lot* for newish files. -}
-getKeyLog :: Key -> [CommandParam] -> Annex ([RefChange Key], IO Bool)
+getKeyLog :: Key -> [CommandParam] -> Annex ([LoggedFileChange Key], IO Bool)
 getKeyLog key os = do
 	top <- fromRepo Git.repoPath
 	p <- liftIO $ relPathCwdToFile top
@@ -287,7 +287,7 @@ getKeyLog key os = do
 	let logfile = p P.</> locationLogFile config key
 	getGitLogAnnex [fromRawFilePath logfile] (Param "--remove-empty" : os)
 
-getGitLogAnnex :: [FilePath] -> [CommandParam] -> Annex ([RefChange Key], IO Bool)
+getGitLogAnnex :: [FilePath] -> [CommandParam] -> Annex ([LoggedFileChange Key], IO Bool)
 getGitLogAnnex fs os = do
 	config <- Annex.getGitConfig
 	let fileselector = \_sha f ->