ticket: codecov/engineering-team#1622

These changes introduce a new capabiliy to `ComparisonProxy`. Namely, to get the
difference in the number of uploads reported per flag between HEAD and BASE.

The idea is to be able to give more actionable message in the PR comment on the following scenario:
1. I have uploaded 5 reports to my base commit
2. I upload 1 report to head commit
3. I have NOT covered all lines of my patch
4. I'm seeing project coverage go WAY down

Because typically coverage is uploaded via CI, and CI doesn't change often, a different
number of reports present could indicate issues in the coverage results.

We have to ignore carryforward sessions because different commits might upload different
sets of reports.

There are 2 functions so that we can also get the _total_ number of reports uploaded per flag, not only the diff

bundle tests together.
slight refactor of dict buildup