GH perf bot missing perf builds message completely for rollup

[klensy]

rust-lang/rust#100977 is rollup of 8, but no perf builds message, like in rust-lang/rust#100963 (comment).

Probably because of rustc_codegen_cranelift sync with lots things in it.

Similar to #1390

[rylev]

👀 The webhook was successful which means the error happened when we're processing it in the background. I'll need to jump into the logs to find out more.

[rylev]

Here's the successfully created unrolled PR comment: rust-lang/rust#100977 (comment).

You'll notice that two of the PRs could not be unrolled due to merge conflicts (which is why this was originally not working).

[rylev]

Looking at things more closely, it turns out that the mechanism mistakenly found 2 rollup merge commits because one of the rollup PRs itself contained rollup merge commits. Unfortunately, I'm pretty sure there's no way for us to detect that in our current approach due to the issue found in #1390.

The way to solve this is to follow the commit tree directly instead of using the Push payload to try to determine the right set of commits.