Improve rust-timer rollup messages

[Rémy Rakic]

We can improve the messages rust-timer posts on rollup with the list of perf builds for the PRs it contains (random example here).

Here are a few thoughts encountered while using the feature:

• the PRs look to be ordered by their number in descending order, which is usually the opposite of what is listed in the OP, and harder to scan.
• the table only contains the PR number but not its title, and makes it a bit hard to scan: to see which build to check, the title of the PRs is usually key when comparing the regression on a given benchmark. For example: the detailed query view of a given benchmark shows a regression in the borrowck query, so we'll look in the list of rolled up PRs for one that changed that area of the compiler. We have to read the title in the OP, remember the number, and go back to the timer message to find the sha1.
• copy-pasting is hard, the SHA1s are linked by GH and one needs to remember to paste without formatting to launch a perf run. The full command could be available in the message to be easy to copy paste.