github: this branch is N commits behind

Don't try it with the github site (it's possible but a bad idea). Instead use the "clean" way and do this from inside the dir with your git-repo:

1. Tell your repository about the original one:

   git remote add upstream git://github.com/ORIGINAL_OWNER/ORIGINAL_REPOSITORY.git

2. Get the original branches: git fetch upstream

3. Pull in the original data in to your branch: git pull upstream master

4. Push this to your github repository: git push

Update: you can now perform this action via the GitHub frontend's Fetch Upstream button - see the Release Notes here:

You can now use the web UI to synchronize an out of date branch of a fork with its upstream branch. If there are no merge conflicts between the branches, the fork's branch is updated either by fast-forwarding or by merging from the upstream's branch. If there are conflicts, you will be prompted to open a pull request to resolve.

Old Answer:

Create a pull request that merges OtherOne:master into your forked repo, then merge it. That should remove the message and ensure that the two repos are indeed synchronized.