annex.backend is the new name for what was annex.backends

It takes a single key-value backend, rather than the unncessary and confusing list.
The old option still works if set.

Simplified some old old code too.

This commit was sponsored by Thomas Hochstein on Patreon.

Types/GitConfig.hs

@@ -48,7 +48,7 @@ data GitConfig = GitConfig
 	, annexNumCopies :: Maybe NumCopies
 	, annexDiskReserve :: Integer
 	, annexDirect :: Bool
-	, annexBackends :: [String]
+	, annexBackend :: Maybe String
 	, annexQueueSize :: Maybe Int
 	, annexBloomCapacity :: Maybe Int
 	, annexBloomAccuracy :: Maybe Int
@@ -98,7 +98,12 @@ extractGitConfig r = GitConfig
 	, annexDiskReserve = fromMaybe onemegabyte $
 		readSize dataUnits =<< getmaybe (annex "diskreserve")
 	, annexDirect = getbool (annex "direct") False
-	, annexBackends = getwords (annex "backends")
+	, annexBackend = maybe
+		-- annex.backends is the old name of the option, still used
+		-- when annex.backend is not set.
+		(headMaybe $ getwords (annex "backends"))
+		Just
+		(getmaybe (annex "backend"))
 	, annexQueueSize = getmayberead (annex "queuesize")
 	, annexBloomCapacity = getmayberead (annex "bloomcapacity")
 	, annexBloomAccuracy = getmayberead (annex "bloomaccuracy")