addon commands

Seems only fair, that, like git runs git-annex, git-annex runs
git-annex-foo.

Implementation relies on O.forwardOptions, so that any options are passed
through to the addon program. Note that this includes options before the
subcommand, eg: git-annex -cx=y foo

Unfortunately, git-annex eats the --help/-h options.
This is because it uses O.hsubparser, which injects that option into each
subcommand. Seems like this should be possible to avoid somehow, to let
commands display their own --help, instead of the dummy one git-annex
displays.

The two step searching mirrors how git works, it makes finding
git-annex-foo fast when "git annex foo" is run, but will also support fuzzy
matching, once findAllAddonCommands gets implemented.

This commit was sponsored by Dr. Land Raider on Patreon.

CmdLine/GlobalSetter.hs

@@ -1,6 +1,6 @@
 {- git-annex global options
  -
- - Copyright 2015 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
   -}
@@ -19,6 +19,7 @@ globalFlag setter = flag' (DeferredParse setter)
 globalSetter :: (v -> Annex ()) -> Parser v -> GlobalOption
 globalSetter setter parser = DeferredParse . setter <$> parser
 
-combineGlobalOptions :: [GlobalOption] -> Parser GlobalSetter
-combineGlobalOptions l = DeferredParse . mapM_ getParsed
+parserGlobalOptions :: [GlobalOption] -> Parser GlobalSetter
+parserGlobalOptions [] = DeferredParse <$> pure noop
+parserGlobalOptions l = DeferredParse . mapM_ getParsed
 	<$> many (foldl1 (<|>) l)