Verify variations of branch protection that are in play when merging a
pull request as:
* instance admin
* repository admin / owner
* user with write permissions on the repository
In all cases the result is expected to be the same when merging
the pull request via:
* API
* web
Although the implementations are different.
(cherry picked from commit 793421bf59)
Conflicts:
tests/integration/pull_merge_test.go
trivial context conflict
