This is now supported by Github in a bit of a round-about way (post here). Steps:

1. Open repository Settings

2. Rules → Rulesets

3. New Ruleset → New Branch Ruleset

4. Ruleset Name: “Linear History”

5. Enforcement Status: Active

6. Add Target → Include all branches

7. Branchset rules → Require linear history (only)

8. Create

The default when a GitHub branch is out of date will now be "Rebase branch".

NOTE: You may need to click "refresh" in the browser for each branch created prior to the above ruleset in order to see the button update. If you click "Update branch" instead of refreshing the page, you will cause that branch to no longer be updatable from the GitHub UI.