Support pull_request_target

[Ana06]

pull_request_target is granted a read/write repository token and runs in the context of the base of the pull request. It allows to create workflows that label and comment on pull requests based on the contents of the event payload. The get-changed-files action is useful for some of those actions and the code works for this case with almost no modification.

You can see it working here:

• https://github.com/Ana06/capa/runs/2624372214
• Update the README but not the CHANGELOG Ana06/capa#28

You can try it using:

- name: Get changed files
  id: files
  uses: Ana06/get-changed-files@master

Example: Ana06/capa@ede0ddf

Supersedes #16 and #13 (I think dist needed to be updated in the first PR and it is not needed to delete the "head commit is ahead of the base commit" check in the other).

A release with this change would be appreciated. 😄

[Ana06]

It seems this project is not maintained anymore or there is no interest on adding this. I have released Ana06/get-changed-files@v1.1 which includes this change, #27 and fixes the tests. Feel free to cherry-pick the commits if you want.