GitHub has added an ability to allow users with write permissions to the branch against which the PR is being raised to have write rights to the originating branch.
This is an opt-out feature, i.e.
Only pull request creators can give upstream repository maintainers, or those with push access to the upstream repository, permission to make commits to their pull request's compare branch.
[..]
Pull request creators can give these permissions on each of their pull requests when they initially create a pull request from a fork or after they have created the 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.
User epoberezkin created a pull request against gajus:master
. The origin of this request is epoberezkin:precompile-schemas
. Therefore, as a user with write-permissions to gajus:master
I can push to epoberezkin:precompile-schemas
branch and these changes will be reflected in the 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
# Make changes, commit changes.
git push epoberezkin HEAD:precompile-schemas