git-annex unused --from remote skips its git-remote-annex keys

This turns out to only be necessary is edge cases. Most of the
time, git-annex unused --from remote doesn't see git-remote-annex keys
at all, because it does not record a location log for them.

On the other hand, git-annex unused does find them, since it does not
rely on the location log. And that's good because they're a local cache
that the user should be able to drop.

If, however, the user ran git-annex unused and then git-annex move
--unused --to remote, the keys would have a location log for that
remote. Then git-annex unused --from remote would see them, and would
consider them unused. Even when they are present on the special remote
they belong to. And that risks losing data if they drop the keys from
the special remote, but didn't expect it would delete git branches they
had pushed to it.

So, make git-annex unused --from skip git-remote-annex keys whose uuid
is the same as the remote.

Backend/GitRemoteAnnex.hs

@@ -14,6 +14,7 @@ module Backend.GitRemoteAnnex (
 	backends,
 	genGitBundleKey,
 	genManifestKey,
+	isGitRemoteAnnexKey,
 ) where
 
 import Annex.Common
@@ -24,9 +25,10 @@ import Utility.Metered
 import qualified Backend.Hash as Hash
 
 import qualified Data.ByteString.Short as S
+import qualified Data.ByteString.Char8 as B8
 
 backends :: [Backend]
-backends = [gitbundle]
+backends = [gitbundle, gitmanifest]
 
 gitbundle :: Backend
 gitbundle = Backend
@@ -44,6 +46,19 @@ gitbundle = Backend
 		Hash.cryptographicallySecure hash
 	}
 
+gitmanifest :: Backend
+gitmanifest = Backend
+	{ backendVariety = GitManifestKey
+	, genKey = Nothing
+	, verifyKeyContent = Nothing
+	, verifyKeyContentIncrementally = Nothing
+	, canUpgradeKey = Nothing
+	, fastMigrate = Nothing
+	, isStableKey = const True
+	, isCryptographicallySecure = False
+	, isCryptographicallySecureKey = const $ pure False
+	}
+
 -- git bundle keys use the sha256 hash.
 hash :: Hash.Hash
 hash = Hash.SHA2Hash (HashSize 256)
@@ -72,5 +87,18 @@ genGitBundleKey remoteuuid file meterupdate = do
 genManifestKey :: UUID -> Key
 genManifestKey u = mkKey $ \kd -> kd
 	{ keyName = S.toShort (fromUUID u)
-	, keyVariety = OtherKey "GITMANIFEST"
+	, keyVariety = GitManifestKey
 	}
+
+{- Is the key a manifest or bundle key that belongs to the special remote
+ - with this uuid? -}
+isGitRemoteAnnexKey :: UUID -> Key -> Bool
+isGitRemoteAnnexKey u k = 
+	case fromKey keyVariety k of
+		GitBundleKey -> sameuuid $
+			-- Remove the checksum that comes after the UUID.
+			B8.dropEnd 1 . B8.dropWhileEnd (/= '-')
+		GitManifestKey -> sameuuid id
+		_ -> False
+  where
+	sameuuid f = fromUUID u == f (S.fromShort (fromKey keyName k))