add startAction parameter for KeySha

I have a use planned for this in Command.Migrate.

Sponsored-by: unqueued on Patreon

Command/Get.hs

@@ -41,7 +41,7 @@ seek :: GetOptions -> CommandSeek
 seek o = startConcurrency transferStages $ do
 	from <- maybe (pure Nothing) (Just <$$> getParsed) (getFrom o)
 	let seeker = AnnexedFileSeeker
-		{ startAction = start o from
+		{ startAction = const $ start o from
 		, checkContentPresent = Just False
 		, usesLocationLog = True
 		}