[FIX] test_pylint: Retrieve correctly changed modules on GitLab CI

[luisg123v]

Currently, when tests are run on GitLab CI, list of changed modules are not retrieved correctly. They are computed comparing changes against origin/base_branch (e.g. origin/11.0). However, on GitLab CI, origin doesn't contain the base branch, but the head branch (e.g. vauxoo-dev/maintainer-quality-tools, not Vauxoo/maintainer-quality-tools).

This commit computes changed files taking changes from the most recent commit. Since there's no way to know the base/target branch, thus there's no way to know for sure what files were modified, our best bet is to compare changes against HEAD~1, and hope the MR doesn't contain more than one commit.

Closes #271

Dummy MRs:

• Qualifirst: https://git.vauxoo.com/vauxoo/qualifirst/merge_requests/308
• Inteco: https://git.vauxoo.com/vauxoo/inteco/merge_requests/417

[luisg123v]

@moylop260
Could you review this, please?

Note:
Dummy MRs are red, but they are expected errors, both contain lint issues in non-excluded modules, so this is working as it should.