gitlab-advanced-sast-cpp: support diff-based scanning
Problem to solve
Faster Advanced SAST: Diff-based scanning in MRs (&16790 - closed) • Shao Ming Tan • 18.5 was closed before (or around the same time as) the C/C++ analyzer Beta, so diff-based scanning was not introduced for C/C++. We now need to add support for it in gitlab-advanced-sast-cpp.
Reference
/cc @cwidstrom @efeller @smtan