oldest_commits_with: fix caching

[emillon]

The current version has an important issue: oldest_commit_with always returns the commit equals to from. In other words, the most recent commit of opam-repository. This is correct but destroys caching because every opam-repository push will trigger a rebuild of all ocaml-ci repositories.

The reason is that the paths argument to that function is []. In turn, this is caused by is_path_in_repo always returning false. There are two issues in that function:

• the string passed as a second argument to / is not a valid segment, so it raises
• the clone is a bare one, so it does not contain any files

Removing this filtering makes the function return an older commit (and it is possible to observe that paths are being passed to git log, for example using execsnoop).

[emillon]

(note that I'm not sure about the correctness of the fix - in particular whether some partitioning is necessary)

[moyodiallo]

I introduced this bug during the merge of the 2 solvers, from OCaml-multicore-CI and OCaml-CI. The differences are introduced by this commit https://github.com/ocurrent/ocaml-multicore-ci/commit/975a2605ea8b6119fa7fae3b512ede581728215c.

This PR could be a good start to harmonize things or doing them better.