instantiate remotes that are behind a proxy remote

Untested, but this should be close to working. The proxied remotes have
the same url but a different uuid. When talking to current
git-annex-shell, it will fail due to a uuid mismatch. Once it supports
proxies, it will know that the presented uuid is for a remote that it
proxies for.

The check for any git config settings for a remote with the same name as
the proxied remote is there for several reasons. One is security:
Writing a name to the proxy log should not cause changes to
how an existing, configured git remote operates in a different clone of
the repo.

It's possible that the user has been using a proxied remote, and decides
to set a git config for it. We can't tell the difference between that
scenario and an evil remote trying to eg, intercept a file upload
by replacing their remote with a proxied remote.

Also, if the user sets some git config, does it override the config
inherited from the proxy remote? Seems a difficult question. Luckily,
the above means we don't need to think through it.

This does mean though, that in order for a user to change the config of
a proxy remote, they have to manually set its annex-uuid and url, as
well as the config they want to change. They may also have to set any of
the inherited configs that they were relying on.

Annex/UUID.hs

@@ -6,7 +6,7 @@
  - UUIDs of remotes are cached in git config, using keys named
  - remote.<name>.annex-uuid
  -
- - Copyright 2010-2016 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2024 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -15,6 +15,7 @@
 
 module Annex.UUID (
 	configkeyUUID,
+	configRepoUUID,
 	getUUID,
 	getRepoUUID,
 	getUncachedUUID,
@@ -47,6 +48,9 @@ import Data.String
 configkeyUUID :: ConfigKey
 configkeyUUID = annexConfig "uuid"
 
+configRepoUUID :: Git.Repo -> ConfigKey
+configRepoUUID r = remoteAnnexConfig r "uuid"
+
 {- Generates a random UUID, that does not include the MAC address. -}
 genUUID :: IO UUID
 genUUID = toUUID <$> U4.nextRandom
@@ -82,7 +86,7 @@ getRepoUUID r = do
 	updatecache u = do
 		g <- gitRepo
 		when (g /= r) $ storeUUIDIn cachekey u
-	cachekey = remoteAnnexConfig r "uuid"
+	cachekey = configRepoUUID r
 
 removeRepoUUID :: Annex ()
 removeRepoUUID = do