fix false positive in export conflict detection

Like the earlier fixed one in Command.Export, it occurred when the same
tree was exported by multiple clones. Previous fix was incomplete since
several other places looked at the list of exported trees to detect when
there was an export conflict. Added a single unified function to avoid
missing any places it needed to be fixed.

This commit was sponsored by mo on Patreon.

Remote/Helper/Export.hs

@@ -188,7 +188,7 @@ adjustExportable r = case M.lookup "exporttree" (config r) of
 			, checkPresentCheap = False
 			, mkUnavailable = return Nothing
 			, getInfo = do
-				ts <- map (fromRef . exportedTreeish)
+				ts <- map fromRef . exportedTreeishes
 					<$> getExport (uuid r)
 				is <- getInfo r
 				return (is++[("export", "yes"), ("exportedtree", unwords ts)])