[Utils] Add ability to configure git-llvm-push from .gitconfig (#187398)
This lets someone set git config options at whatever scope (per-repo,
global, etc.) for the options that they care about. This provides
similar functionality to just wrapping the script in a shell script with
one's desired options without the need to do that.
We need to be careful about how when we get the flags and how to execute
the git command to get the flags. For now, we do this before normal
argument parsing and fail silently to avoid printing output if someone
passes something like --quiet through the git config. This means options
like --verbose and --dry-run don't work for this specific command, but I
think that is a reasonable tradeoff.