Fetch deeper the tested branch before fetching master

What does this MR do?

This merge request ensures that the tested branch is fetched deeper than the runner's default which is 20 commits, otherwise, if the branch is more than 20 commits ahead of master, no merge base could ever be found, and the job would fail because "<branch> is too far behind master, please rebase it!" (see https://gitlab.com/gitlab-org/gitlab-ce/builds/11000287), when the actual situation is that <branch> is too far ahead of master and it should be fetched deeper.

This merge request also improves the commands logging and the final messages.

Are there points in the code the reviewer needs to double check?

I initially wanted to add specs using Rugged etc. but I don't have the time right now. :(