Drone CI: Wrong branch reported for PRs

[abrain]

When uploading the coverage report for a Pull Request using Drone CI, the result is attributed to the target branch of the Pull Request.

Situation: I want to merge the branch feature/foo into the branch develop of the same repo and open a PR for it. Whenever the PR gets tested, the results on Code Climate for develop are overwritten with the results from the PR.

On Drone CI, the test-reporter tries to determine the branch from the DRONE_BRANCH environment variable.

test-reporter/env/git.go

Line 159 in f002281

var gitBranchVars = []string{"GIT_BRANCH", "APPVEYOR_REPO_BRANCH", "BRANCH_NAME", "BUILDKITE_BRANCH", "CIRCLE_BRANCH", "CI_BRANCH", "CI_BUILD_REF_NAME", "DRONE_BRANCH", "HEROKU_TEST_RUN_BRANCH", "TRAVIS_PULL_REQUEST_BRANCH", "TRAVIS_BRANCH", "WERCKER_GIT_BRANCH", "CI_COMMIT_REF_NAME", "BITRISE_GIT_BRANCH"}

For push events this is fine, but for Pull Requests it contains the target branch of the PR, not the source branch. As far as I can tell, there is no single environment variable that would fit the purpose. As a workaround, I use a script to set GIT_BRANCH to the correct branch before uploading.