Support for partial Codeclimate analysis

Proposal

A customer (internal link) Has proposed adding the ability to do partial code analysis with CodeClimate.

Currently they achieve this through the following:

By the way the we manually forced partial code-analysis with the following scenario:

The master branch always perform a full scan at commit time
Secondary branches always perform a partial analysis when triggered by MR
To execute "partial parsing" we added a before_script to the code_quality job that performs a git diff between the current branch and the target branch and deletes unmodified files - this produced a report of 256kb instead of 40mb containing all the needed findings.

This leads to unintended consequences that could be worked around by implementing this feature in GitLab.

It doesn't know that the report is only partial, so it shows all previous findings as fixed
If you run a partial report on a branch that has another partial report, you will get an inconsistent diff report and there is no way to make the comparator consider the parent branch report or trigger a full cq pipeline just to get it to compare correctly.