xcontext: Add links to Go issues regarding context.Merge

https://github.com/golang/go/issues/36503
https://github.com/golang/go/issues/36448

They are there for 6 months already without being considered for real
and so the progress is very unlikely, but still...

xcontext/xcontext.go

-// Copyright (C) 2017-2019  Nexedi SA and Contributors.
+// Copyright (C) 2017-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -58,6 +58,9 @@ import (
 // XXX if we could change std context, then Merge could work by simply creating
 // cancelCtx and registering it to parent1 and parent2.
 //
+//	https://github.com/golang/go/issues/36503
+//	https://github.com/golang/go/issues/36448
+//
 // For the reference: here is how it is done in pygolang:
 //
 //	https://lab.nexedi.com/kirr/pygolang/blob/d3bfb1bf/golang/context.py#L115-130