Support setting the github.head_ref branch for a Pull Request

[donovanmuller]

Currently this action returns a branch like the below example when triggered from a pull_Request:

BRANCH_NAME: 3/merge

The branch returned from the expression github.head_ref would be the expected value. It would be great if this action could support this.

[EthanSK]

I made an action that works on pull_request too https://github.com/EthanSK/git-branch-name-action