started converting to use optparse-applicative

This is a work in progress. It compiles and is able to do basic command
dispatch, including git autocorrection, while using optparse-applicative
for the core commandline parsing.

* Many commands are temporarily disabled before conversion.
* Options are not wired in yet.
* cmdnorepo actions don't work yet.

Also, removed the [Command] list, which was only used in one place.

Command/DiffDriver.hs

@@ -13,10 +13,10 @@ import Annex.Content
 import Annex.Link
 import Git.Types
 
-cmd :: [Command]
-cmd = [dontCheck repoExists $
+cmd :: Command
+cmd = dontCheck repoExists $
 	command "diffdriver" ("[-- cmd --]") seek
-		SectionPlumbing "external git diff driver shim"]
+		SectionPlumbing "external git diff driver shim"
 
 seek :: CommandSeek
 seek = withWords start