`github retarget-pr` should update annotation for the branch

[PawelLipski]

I've got sth like

 main
  │
  └─FOO-138  PR #32
    │
    └─refactor/FOO-131-subnet-typo  PR #33 WRONG PR BASE or MACHETE PARENT? PR has 'main'

Then I've run git machete github retarget-pr while on refactor/FOO-131-subnet-typo:

$ git machete github retarget-pr 
The base branch of PR #33 has been switched to FOO-138

But the annotation hasn't changed... would be good if it was overwritten on retarget, just as with create-pr