add equivilant key log for VURL keys

When downloading a VURL from the web, make sure that the equivilant key
log is populated.

Unfortunately, this does not hash the content while it's being
downloaded from the web. There is not an interface in Backend currently
for incrementally hash generation, only for incremental verification of an
existing hash. So this might add a noticiable delay, and it has to show
a "(checksum...") message. This could stand to be improved.

But, that separate hashing step only has to happen on the first download
of new content from the web. Once the hash is known, the VURL key can have
its hash verified incrementally while downloading except when the
content in the web has changed. (Doesn't happen yet because
verifyKeyContentIncrementally is not implemented yet for VURL keys.)

Note that the equivilant key log file is formatted as a presence log.
This adds a tiny bit of overhead (eg "1 ") per line over just listing the
urls. The reason I chose to use that format is it seems possible that
there will need to be a way to remove an equivilant key at some point in
the future. I don't know why that would be necessary, but it seemed wise
to allow for the possibility.

Downloads of VURL keys from other special remotes that claim urls,
like bittorrent for example, does not popilate the equivilant key log.
So for now, no checksum verification will be done for those.

Sponsored-by: Nicholas Golder-Manning on Patreon

Logs.hs

@@ -1,6 +1,6 @@
 {- git-annex log file names
  -
- - Copyright 2013-2023 Joey Hess <id@joeyh.name>
+ - Copyright 2013-2024 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -35,7 +35,9 @@ getLogVariety config f
 	| isRemoteStateLog f = Just NewUUIDBasedLog
 	| isRemoteContentIdentifierLog f = Just NewUUIDBasedLog
 	| isRemoteMetaDataLog f = Just RemoteMetaDataLog
-	| isMetaDataLog f || f `elem` otherTopLevelLogs = Just OtherLog
+	| isMetaDataLog f
+		|| f `elem` otherTopLevelLogs
+		|| isEquivilantKeyLog f = Just OtherLog
 	| otherwise = (LocationLog <$> locationLogFileKey config f)
 		<|> (ChunkLog <$> extLogFileKey chunkLogExt f)
 		<|> (UrlLog  <$> urlLogFileKey f)
@@ -70,6 +72,7 @@ keyLogFiles config k =
 	, remoteMetaDataLogFile config k
 	, remoteContentIdentifierLogFile config k
 	, chunkLogFile config k
+	, equivilantKeysLogFile config k
 	] ++ oldurlLogs config k
 
 {- All uuid-based logs stored in the top of the git-annex branch. -}
@@ -208,6 +211,18 @@ chunkLogFile config key =
 chunkLogExt :: S.ByteString
 chunkLogExt = ".log.cnk"
 
+{- The filename of the equivilant keys log for a given key. -}
+equivilantKeysLogFile :: GitConfig -> Key -> RawFilePath
+equivilantKeysLogFile config key = 
+	(branchHashDir config key P.</> keyFile key)
+		<> equivilantKeyLogExt
+
+equivilantKeyLogExt :: S.ByteString
+equivilantKeyLogExt = ".log.ek"
+
+isEquivilantKeyLog :: RawFilePath -> Bool
+isEquivilantKeyLog path = equivilantKeyLogExt `S.isSuffixOf` path
+
 {- The filename of the metadata log for a given key. -}
 metaDataLogFile :: GitConfig -> Key -> RawFilePath
 metaDataLogFile config key =