let Remote.availability return Unavilable

This is groundwork for making special remotes like borg be skipped by
sync when on an offline drive.

Added AVAILABILITY UNAVAILABLE reponse and the UNAVAILABLERESPONSE extension
to the external special remote protocol. The extension is needed because
old git-annex, if it sees that response, will display a warning
message. (It does continue as if the remote is globally available, which
is acceptable, and the warning is only displayed at initremote due to
remote.name.annex-availability caching, but still it seemed best to make
this a protocol extension.)

The remote.name.annex-availability git config is no longer used any
more, and is documented as such. It was only used by external special
remotes to cache the availability, to avoid needing to start the
external process every time. Now that availability is queried as an
Annex action, the external is only started by sync (and the assistant),
when they actually check availability.

Sponsored-by: Nicholas Golder-Manning on Patreon

Remote/Git.hs

@@ -1,6 +1,6 @@
 {- Standard git remotes.
  -
- - Copyright 2011-2021 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2023 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -11,7 +11,6 @@
 module Remote.Git (
 	remote,
 	configRead,
-	repoAvail,
 	onLocalRepo,
 ) where
 
@@ -210,7 +209,7 @@ gen r u rc gc rs
 			, readonly = Git.repoIsHttp r
 			, appendonly = False
 			, untrustworthy = False
-			, availability = availabilityCalc r
+			, availability = repoAvail r
 			, remotetype = remote
 			, mkUnavailable = unavailable r u rc gc rs
 			, getInfo = gitRepoInfo new
@@ -233,20 +232,24 @@ unavailable r = gen r'
 		_ -> r -- already unavailable
 
 {- Checks relatively inexpensively if a repository is available for use. -}
-repoAvail :: Git.Repo -> Annex Bool
+repoAvail :: Git.Repo -> Annex Availability
 repoAvail r 
-	| Git.repoIsHttp r = return True
+	| Git.repoIsHttp r = return GloballyAvailable
 	| Git.GCrypt.isEncrypted r = do
 		g <- gitRepo
 		liftIO $ do
 			er <- Git.GCrypt.encryptedRemote g r
 			if Git.repoIsLocal er || Git.repoIsLocalUnknown er
-				then catchBoolIO $
-					void (Git.Config.read er) >> return True
-				else return True
-	| Git.repoIsUrl r = return True
-	| Git.repoIsLocalUnknown r = return False
-	| otherwise = liftIO $ isJust <$> catchMaybeIO (Git.Config.read r)
+				then checklocal er
+				else return GloballyAvailable
+	| Git.repoIsUrl r = return GloballyAvailable
+	| Git.repoIsLocalUnknown r = return Unavailable
+	| otherwise = checklocal r
+  where
+	checklocal r' = ifM (liftIO $ isJust <$> catchMaybeIO (Git.Config.read r'))
+		( return LocallyAvailable
+		, return Unavailable
+		)
 
 {- Tries to read the config for a specified remote, updates state, and
  - returns the updated repo. -}