export appendonly support

Make `git annex export` check appendonly when removing a file from an
export, and not update the location log, since the remote still contains
the content.

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

Remote/Bup.hs

@@ -79,7 +79,7 @@ gen r u c gc = do
 		, remotetype = remote
 		, availability = if bupLocal buprepo then LocallyAvailable else GloballyAvailable
 		, readonly = False
-		, appendonly = True
+		, appendonly = False
 		, mkUnavailable = return Nothing
 		, getInfo = return [("repo", buprepo)]
 		, claimUrl = Nothing