Bench old commits

[ElectreAAS]

Fixes #379.

• pipeline/lib/commits.ml contains the important part: the actual git command to run to traverse the commit history.
• pipeline/libs/models.ml[i] contains the necessary two new functions that query the database, to make sure we don't benchmark twice the same commit.

~~Note: This PR requires some changes to the API of ocurrent (namely this PR: https://github.com/ocurrent/ocurrent/pull/442), and hence is in draft mode to make sure it doesn't get merged before the PR on ocurrent is properly reviewed. It is, however, ready to be reviewed in the current-bench context. The API changes in ocurrent are trivials from our point of view.~~

Edit: ocurrent/ocurrent#442 has been merged, this is ready for review

[art-w]

Thanks! This was a much awaited feature!

A couple of thoughts which prevents this from going live:

• Can we disable it for repos that use a cron schedule? (ocaml/ocaml runs its benchmarks every week, precisely to avoid benchmarking too many commits)
• Is the commit order preserved when plotted on a graph?
• How do we get rid of old commits that were removed? (e.g. following a PR rebase)

[ElectreAAS]

• Can we disable it for repos that use a cron schedule? (ocaml/ocaml runs its benchmarks every week, precisely to avoid benchmarking too many commits)

I have no idea

• Is the commit order preserved when plotted on a graph?

I also have no idea, but it seems testable. I don't have time to allocate to this nowadays, but someone else could.

• How do we get rid of old commits that were removed? (e.g. following a PR rebase)

This PR does not take that scenario into account.