better handling of finishing up incomplete incremental verify

Now it's run in VerifyStage.

I thought about keeping the file handle open, and resuming reading where
tailVerify left off. But that risks leaking open file handles, until the
GC closes them, if the deferred verification does not get resumed. Since
that could perhaps happen if there's an exception somewhere, I decided
that was too unsafe.

Instead, re-open the file, seek, and resume.

Sponsored-by: Dartmouth College's DANDI project

Types/Remote.hs

@@ -2,7 +2,7 @@
  -
  - Most things should not need this, using Types instead
  -
- - Copyright 2011-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -41,6 +41,7 @@ import Types.NumCopies
 import Types.Export
 import Types.Import
 import Types.RemoteConfig
+import Types.Backend (IncrementalVerifier)
 import Config.Cost
 import Utility.Metered
 import Git.Types (RemoteName)
@@ -203,7 +204,9 @@ data Verification
 	| MustVerify
 	-- ^ Content likely to have been altered during transfer,
 	-- verify even if verification is normally disabled
-	deriving (Show)
+	| IncompleteVerify IncrementalVerifier
+	-- ^ Content was partially verified during transfer, but
+	-- the verification is not complete.
 
 unVerified :: Monad m => m a -> m (a, Verification)
 unVerified a = do