GITHUB_SHA is not in GITHUB_REF

[phadej]

I'm not sure where to report these issues so I do it here.

GITHUB_SHA is supposed to be a commit on GITHUB_REF, yet recently this doesn't seem to be true anymore:

E.g. I have a build with

• GITHUB_SHA 85d87de310300c5d60d59e70aa477abc6e97310e
• GITHUB_REF refs/pull/6944/merge

The tip of the latter is however 87a7262c4e7cbc8997fb181c33232db98b8a062e

commit 87a7262c4e7cbc8997fb181c33232db98b8a062e (temp)
Merge: 2d7885397 5c1cb0a57
Author: Oleg Grenrus <[email protected]>
Date:   Tue Jul 7 15:23:57 2020 +0200

    Merge 5c1cb0a57ac08b4ba09bec93757d788ae5e76f3d into 2d7885397012c2eb8faf068d3a2a57bd7b7e7266

and

commit 85d87de310300c5d60d59e70aa477abc6e97310e
Merge: 2d7885397 5c1cb0a57
Author: Oleg Grenrus <[email protected]>
Date:   Tue Jul 7 13:23:56 2020 +0000

    Merge 5c1cb0a57ac08b4ba09bec93757d788ae5e76f3d into 2d7885397012c2eb8faf068d3a2a57bd7b7e7266

This breaks my workaround for checkout@v2 as it doesn't create .git with older versions of git #170. I'm fine with having slow checkout with older git.

[ericsciple]

@andymckay