Fix reading config file when pull request from forked repository

[nowsprinting]

Changes

• Using base.ref in read config file when pull request from forked repo
• Add test
• Add logging

Reason

• When pull request from forked repo, head.ref is fork-repo's ref. It does not exist in base-repo.

refs #25

[nowsprinting]

Inner pull request example (regression test)
nowsprinting/pr-labeler-action-test#4

Forked pull request example (fixed this issue, read config from base.ref)
nowsprinting/pr-labeler-action-test#5

[TimonVS]

Thanks for the contributions and a special thank you for including test results 🙌

I suppose this is necessary because it doesn't have permissions to read the fork's config file when using the pull_request_target event?

[nowsprinting]

I suppose this is necessary because it doesn't have permissions to read the fork's config file when using the pull_request_target event?

No. This problem (refs #25 ) has two steps.

1. Readonly GITHUB_TOKEN and not read secret on pull_request trigger
   So pr-labeler-action is not work. (See: Forked pull request example (pull_request trigger) nowsprinting/pr-labeler-action-test#6 )
   Require using pull_request_target trigger.

2. fork-repo's head.ref is not exist in base-repo
   Using base.ref.
   Fixed it in this PR.