formulae_detect: use the first parent of GITHUB_SHA

[carlocab]

Unfortunately, GITHUB_SHA is the same as origin/master inside a
merge group, so we end up with diff_start_sha1 and diff_end_sha1
pointing to the same (merge) commit.

See, for example: Homebrew/homebrew-core@2ad18dc

[MikeMcQuaid]

Unfortunately, GITHUB_SHA is the same as origin/master inside a
merge group

So origin/master is already set to the commit that will be merged rather than the last commit that was merged? Does this still apply if a git fetch is done?

Regardless: I still don't think this is the correct fix here, I'm afraid.

[carlocab]

So origin/master is already set to the commit that will be merged rather than the last commit that was merged? Does this still apply if a git fetch is done?

It would seem so, yes. Let's try a git fetch first.

Regardless: I still don't think this is the correct fix here, I'm afraid.

Perhaps, but picking up some formulae still seems better than picking up none.

[carlocab]

Ah, wait, no, origin_ref here will be the name of the temporary merge group branch, so it makes sense for that to be the same as GITHUB_SHA, and not be the same as origin/master.

[MikeMcQuaid]

so it makes sense for that to be the same as GITHUB_SHA, and not be the same as origin/master.

Cool. Checking origin/HEAD should be the right fix here, then 👍🏻

[carlocab]

#908