gitaly/config: Add option to ignore gitconfig files
We're deprecating use of gitconfig files which exist in the filesystem
in favor of adding [[git.config]]
entries to Gitaly's config.toml
.
Add a new option that allows distributions to opt-in to this new
behaviour so that they can migrate earlier if they decide to. This will
become the default with release 16.0.
This MR also adds infrastructure to automatically verify whether we read the gitconfig or not as part of our CI to guard against future regressions.
Closes #4242 (closed).