After some research I’ve found this solution:
[the real relative root of any fork](/../../)
It always points to the default branch. For me it’s Ok, so it’s up to you
PS
With such a trick you can also access the following abilities:
[test](/../../tree/test) – link to another branch
[doc/readme.md](/../../edit/master/doc/readme.md) – open in editor
[doc/readme.md](/../../delete/master/doc/readme.md) – ask to delete file
[doc/readme.md](/../../commits/master/doc/readme.md) – history
[doc/readme.md](/../../blame/master/doc/readme.md) – blame mode
[doc/readme.md](/../../raw/master/doc/readme.md) – raw mode (will redirect)
[doc/](/../../new/master/doc/) – ask to create new file
[doc/](/../../upload/master/doc/) – ask to upload file
[find](/../../find/test) – find file