Match commits to PRs by using the extracted PR number

[alice-i-cecile]

As seen in #1518, matching PRs by names is unreliable due to both time windows and renames. The fundamental strategy here is dumb though: we should be able to extract the PR number / link responsible from the commit message and then look it up directly. No inexact matching or expensive "grab all PRs" required.

[mockersf]

it's what I do in the script to cherry pick commits for a release candidate:

bevy-website/content/learn/contribute/project-information/release-process.md

Line 138 in 291bda9

if git log --format=oneline | grep " (#$number)$" > /dev/null; then