get `default_branch` from @payload, use that for the Git.pull#24

Fuzzwah wants to merge 1 commit intogithub-developer:mastergithub-developer/creating-ci-tests-with-the-checks-api:masterfrom

Fuzzwah:patch-2Fuzzwah/creating-ci-tests-with-the-checks-api:patch-2Copy head branch name to clipboard