Allow to change branch of Pull Request (support for rebase with renaming scenario)

As far i know, the Pull Request is simple reference in refs/pulls space. Will be nice to support follow scenario of conflicts resolving:
1. User create Pull Request from branch 'feature' to 'master'.
2. After review branch feature can't be merged cos of conflict
3. User make:

$ git fetch
$ git checkout feature
$ git rebase master
# Resolve conflicts
$ git branch -M feature/1
$ git push origin feature/1

Then (and for now) user must decline old pull request and create new one. But maybe his wand just to change reference to branch of pull request.

This feature look like follow conflict resolution:

$ git fetch
$ git checkout feature
$ git rebase master
# Resolve conflicts
$ git push -f origin feature

Here branch is not renamed, but push -f is used. This maybe dangerous, if branch feature is on work of several developers, not a single one.