fix export subtree reversion

Fix reversion in last release that caused wrong tree to be written to
remote tracking branch after an export of a subtree.

The invariant "commitsha should have the treesha as its tree"
was not met due to a bug. Guarantee it's met by catting the commitsha
to find its actual tree. A little bit slower, but this is not run often.

Command/Export.hs

@@ -235,7 +235,7 @@ fillExport r db newtree mtbcommitsha = do
 		Nothing -> noop
 		Just (tb, commitsha) ->
 			whenM (liftIO $ fromAllFilled <$> takeMVar allfilledvar) $
-				makeRemoteTrackingBranchMergeCommit tb commitsha newtree
+				makeRemoteTrackingBranchMergeCommit tb commitsha
 					>>= setRemoteTrackingBranch tb
 	
 	liftIO $ fromFileUploaded <$> takeMVar cvar