Allow downstream projects configs to be file_config (#1778)
Reapply https://github.com/bazelbuild/continuous-integration/pull/1776
with fixes:
- We need to properly clone the git repository so that we can get
`file_config` at the correct commit.
- Refactored `downstream-root` so that we don't need to get platform
info from task config for cloning the repo, which introduces a
chicken-egg problem.
- Allow suppressing stdout in `clone_git_repository` so that it won't
impact stdout of `project_pipeline`
- Make `validate_config` also work for downstream projects by adding
`git_commit` as a parameter of `get_modified_files`.
5 files changed
