fedcode-next: Improve tracking of fix commits

[pombredanne]

We should support tracking fix commits explicitly to support next steps with reachability analysis. At a high level we need these:

• VCIO-next: Add support to track fix commits: Include commits and patches that fix a vulnerability #207
• VCIO-next: Create improver to find fix commits #1696

Somme issues that support or are related to this are:

• RFC: vulnerability reachability #1313

[TG1999]

Depends on #2152

[ziadhany]

this is done now we have a Improve that collect fix commits from a reference URL.

see:

• Add support for Reference Fix Commits improver #2163