Incorrect warning about outdated upstream commit

[RalfJung]

rustbot claims that my PR is based on a very old upstream commit. That's not correct though, it is based on rust-lang/rust@2b96ddc which isn't that old.

Probably the bot gets confused by this being a subtree sync that contains merge commits.

[RalfJung]

This was implemented in #1942, Cc @xizheyin @Kobzol @Urgau

[Urgau]

Our current logic is too naïve, we only look at the lastest commit date, which is unrealible for subtree.

I had a quick look and I think we would be better off using the "merge base" commit. Which we can get from GitHub compare API. (I tested it on your PR and it seems to give the right commit).

[Kobzol]

Once again, a nice simple heuristic broken by subtrees xD

[xizheyin]

Haha, was there a similar situation before?

[xizheyin]

I didn't figure out why it was giving false warnings. The head commit of rust-lang/rust#141383 (comment) should be here, and according to our implementation, its first parent commit is here and the commit date is 2025-05-21T19:48:09Z.

[Urgau]

@xizheyin The HEAD commit is actually rust-lang/rust@3f0c39d, but it's not the one we are looking for, we are looking at oldest/first one, which is rust-lang/rust@f9a75a0, and it's parent has a commit date of 2025-03-31T (52 days).

[Urgau]

Opened #2005 to fix the issue

[xizheyin]

Oh, I remembered wrong. We were looking for commits.first(). Indeed, the latest method will be more robust.