How to fetch the best common ancestor of the Pull Request with the Rest API ?

[Jgafner]

Hi
I need to make a rest call to our server and to get the commit hash of the best common ancestor, same as running 'git merge-base pr-last-commit target-last-commit'

I know how to get the all commits and check if those are merge commits but how can I know if the merge is from the target or from another branch ? 

I don't/can't to rely on the commit message. 

Thanks 

[Jgafner]

Still did not find a good way to do that