GitHub added the ability to allow users with write permissions to the branch against which the PR is raised in order to have write permissions to the original branch.
This is a failure function, i.e.
Request creators can only provide support to third-party repositories with pulling, or those who have push access to the upstream repository are allowed to commit in the request request comparison branch.
[..]
Request request creators can provide these permissions for each of their pull requests when they initially create a pull request or after they create a pull request.
https://help.github.com/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork/
Example
Lets use this PR as an example, https://github.com/gajus/table/pull/19 .
Epoberezkin created a migration request for gajus:master . The origin of this request is epoberezkin:precompile-schemas . Therefore, as a user with write permissions on gajus:master , I can click on the epoberezkin:precompile-schemas , and these changes will be reflected in PR, i.e.
git clone git@github.com :gajus/table.git cd table git remote add epoberezkin git@github.com :epoberezkin/table.git git fetch epoberezkin git checkout epoberezkin/precompile-schemas
Gajus source share