* [CIFuzz] Fix diffing + Refactor
Make diffing work in two scenarios where it previously failed:
1. Commit fuzzing. In this case, we diff $COMMIT against
$COMMIT^1 because the intent here is to fuzz the commit.
2. Fuzzing PRs that aren't to master. In this case, we previously
were diffing against origin/master. Instead, diff against the local
version of the base repo. This also has the nice effect of handling
PRs that havent pulled from master recently enough.
Also do refactoring.
1. Move code that is different for differenct CI systems to continuous_integration.py
2. Change how configuration in build_fuzzers is handled.
Previously configuration was gotten in build_fuzzers_entrypoint
and passed as individual params. This made code ugly and hard to
read. Instead, move code dealing with config to it's own module
config_utils. This module implements a config class which can
be used to create objects that are passed around to code that needs
it. Making the code much easier to read.
TODO: Move run_fuzzers code to new config system.
jonathanmetzman