implement exporttree=yes configuration

* Only export to remotes that were initialized to support it.
* Prevent storing key/value on export remotes.
* Prevent enabling exporttree=yes and encryption in the same remote.

SetupStage Enable was changed to take the old RemoteConfig.
This allowed only setting exporttree when initially setting up a
remote, and not configuring it later after stuff might already be stored
in the remote.

Went with =yes rather than =true for consistency with other parts of
git-annex. Changed docs accordingly.

This commit was supported by the NSF-funded DataLad project.

Remote/Git.hs

@@ -111,7 +111,7 @@ gitSetup Init mu _ c _ = do
 	if isNothing mu || mu == Just u
 		then return (c, u)
 		else error "git remote did not have specified uuid"
-gitSetup Enable (Just u) _ c _ = do
+gitSetup (Enable _) (Just u) _ c _ = do
 	inRepo $ Git.Command.run
 		[ Param "remote"
 		, Param "add"
@@ -119,7 +119,7 @@ gitSetup Enable (Just u) _ c _ = do
 		, Param $ fromMaybe (giveup "no location") (M.lookup "location" c)
 		]
 	return (c, u)
-gitSetup Enable Nothing _ _ _ = error "unable to enable git remote with no specified uuid"
+gitSetup (Enable _) Nothing _ _ _ = error "unable to enable git remote with no specified uuid"
 
 {- It's assumed to be cheap to read the config of non-URL remotes, so this is
  - done each time git-annex is run in a way that uses remotes.