support fuzzy matching of addon commands

Note this does find things in PATH that are not executable.
Like searchPath use, the executable bit is not checked. Thing is,
there does not seem to be a binding for access(), which would be the
right way to check that the right execute bit is set. Anyway, if it's in
PATH and it's a file, it's probably fine to treat it as something that
was intended to be executable.

This commit was sponsored by Brock Spratlen on Patreon.

CmdLine.hs

@@ -33,8 +33,8 @@ dispatch :: Bool -> Bool -> CmdParams -> [Command] -> [(String, String)] -> IO G
 dispatch addonok fuzzyok allargs allcmds fields getgitrepo progname progdesc =
 	go addonok allcmds $
 		findAddonCommand subcommandname >>= \case
-			Nothing -> go False allcmds noop
-			Just c -> go addonok (c:allcmds) $
+			Just c -> go addonok (c:allcmds) noop
+			Nothing -> go addonok allcmds $
 				findAllAddonCommands >>= \cs ->
 					go False (cs++allcmds) noop
   where
@@ -158,7 +158,20 @@ findAddonCommand (Just subcommandname) =
 	c = "git-annex-" ++ subcommandname
 
 findAllAddonCommands :: IO [Command]
-findAllAddonCommands = return [] -- TODO
+findAllAddonCommands = 
+	filter isaddoncommand
+		. map (\p -> mkAddonCommand p (deprefix p))
+		<$> searchPathContents ("git-annex-" `isPrefixOf`)
+  where
+	deprefix = replace "git-annex-" "" . takeFileName
+	isaddoncommand c
+		-- git-annex-shell
+		| cmdname c == "shell" = False
+		-- external special remotes
+		| "remote-" `isPrefixOf` cmdname c = False
+		-- external backends
+		| "backend-" `isPrefixOf` cmdname c = False
+		| otherwise = True
 
 mkAddonCommand :: FilePath -> String -> Command
 mkAddonCommand p subcommandname = Command