add startAction parameter for KeySha

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

Sponsored-by: unqueued on Patreon

Command/Migrate.hs

@@ -43,7 +43,7 @@ seek o = withFilesInGitAnnex ww seeker =<< workTreeItems ww (migrateThese o)
   where
 	ww = WarnUnmatchLsFiles "migrate"
 	seeker = AnnexedFileSeeker
-		{ startAction = start o
+		{ startAction = const $ start o
 		, checkContentPresent = Nothing
 		, usesLocationLog = False
 		}