implement annex.privateremote and remote.name.private configs

The slightly unusual parsing in Types.GitConfig avoids the need to look
at the remote list to get configs of remotes. annexPrivateRepos combines
all the configs, and will only be calculated once, so it's nice and
fast.

privateUUIDsKnown and regardingPrivateUUID now need to read from the
annex mvar, so are not entirely free. But that overhead can be optimised
away, as seen in getJournalFileStale. The other call sites didn't seem
worth optimising to save a single MVar access. The feature should have
impreceptable speed overhead when not being used.

Git/Remote.hs

@@ -1,6 +1,6 @@
 {- git remote stuff
  -
- - Copyright 2012 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -25,7 +25,11 @@ import Git.FilePath
 
 {- Is a git config key one that specifies the url of a remote? -}
 isRemoteUrlKey :: ConfigKey -> Bool
-isRemoteUrlKey (ConfigKey k) = "remote." `S.isPrefixOf` k && ".url" `S.isSuffixOf` k
+isRemoteUrlKey = isRemoteKey "url"
+
+isRemoteKey :: S.ByteString -> ConfigKey -> Bool
+isRemoteKey want (ConfigKey k) =
+	"remote." `S.isPrefixOf` k && ("." <> want) `S.isSuffixOf` k
 
 {- Get a remote's name from the a config key such as remote.name.url
  - or any other per-remote config key. -}