Creates a singleton class for GitClient rather than relying on creating an instance to
require being passed around throughout its usages.
PR Close#41515
Adds a new folder to dev-infra where shared testing utilities
could be placed in. This commit already adds initial testing
utilities for dealing with the `GitClient` and SemVer versions.
The `GitClient` in the testing utilities simulates actual Git
behavior in a virtual manner. It's not complete at all, but can
be extended based on our needs. The currently implemented commands
are the most basic ones that we'd need for our release tooling.
PR Close#38656
Joey Perrott
Paul Gschwendtner