Improve the benchmarking workflow to generate a commit comment on PRs

[msschwartz21]

I couldn't get the github-actions-benchmark to perform the way I wanted for PRs (although it's working great for generating the charts on github pages). Instead I wrote a slightly more manual action that I think gives us what we want.

The benchmarking action triggered for each PR now does the following:

• Checks if benchmarks have been cached for the base commit and computes and saves them if not
• Runs benchmarking for the current commit
• Generates a table comparing the mean runtime for head and base with the percent change
• Comments on the commit with the comparison table

@bentaculum are there any other statistics that you would want to see in that table?

[bentaculum]

Looks great, just added some small changes to the autogenerated comment table, and added the tabulate dep.

[msschwartz21]

Based on this workflow run, I discovered that the commit comment action does not work on PRs opened from forked repos. This is documented in the action (see the paragraph below the linked table), but I didn't notice when I was setting it up yesterday.

I'll look into the option for configuring pull_request_target to see if that fixes it, but I'm not confident that it will. In the meantime, a short term fix would be to not use forks 😬 I could also change the action to just log the table on PRs coming from forks so at least the action wouldn't fail.

[bentaculum]

Oh, thanks for figuring this out, I was quite baffled why this didn't go through.