fix git-annex sync --content with proxied remotes

Loading the remote list a second time was removing all proxied remotes.
That happened because setting up the proxied remote added some config
fields to the in-memory git config, and on the second load, it saw those
configs and decided not to overwrite them with the proxy.

Now on the second load, that still happens. But now, the proxied
git configs are used to generate a remote same as if those configs were
all set. The reason that didn't happen before was twofold,
the gitremotes cache was not dropped, and the remote's url field was not
set correctly.

The problem with the remote's url field is that while it was marked as
proxy inherited, all other proxy inherited fields are annex- configs.
And the code to inherit didn't work for the url field.

Now it all works, but git-annex sync is left running git push/pull on
the proxied remote, which doesn't work. That still needs to be fixed.

Remote.hs

@@ -448,8 +448,8 @@ claimingUrl' remotefilter url = do
   where
 	checkclaim = maybe (pure False) (`id` url) . claimUrl
 
-{- Is this a remote of a type we can sync with, or a special remote
- - with an annex:: url configured? -}
+{- Is this a remote of a type that git pull and push work with?
+ - That includes special remotes with an annex:: url configured. -}
 gitSyncableRemote :: Remote -> Bool
 gitSyncableRemote r
 	| gitSyncableRemoteType (remotetype r)