[E2E] Add some logging to determine what branches are actually present

What does this MR do and why?

As per [Test] api/12_systems/gitaly/gitaly_mtls_spec.r... (#431474) we are seeing some intermittent spec failures where the spec in question is failing due to what appears to be a case that the main branch isn't present on the server.

Adding some debugging to the spec here to call the branches api so if and when this spec fails again, we'll actually have a picture of what branches are on the server which may shed some light on what might be happening here.

How to set up and validate locally

• Check output of the mtls test job to ensure this logging is present. e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/5512185616

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.