git operations standardized for ci_setup and platform_setup about edk2-pytool-extensions

[Javagedes]

Working on this.

Here is the current situation:

File	Relies on
edk2_git.py	N/A - Basis of repo functionality
repo_resolver.py	edk2_git.py
self_describing_environment.py	PyGit2
git_dependency.py	repo_resolver.py
edk2_ci_setup.py	repo_resolver.py
edk2_setup.py	Uses raw RunCmd git commands

Here is the end state plan:

File	Relies on	Changes
edk2_git.py	N/A - Basis of repo functionality	Use PyGit2 as basis of Repo management
edk2_git_resolver.py	edk2_git.py	repo_resolver + submodule resolve capability
self_describing_environment.py	edk2_git.py	Use edk2_git for worktrees
git_dependency.py	edk2_git_resolver.py	resolve() -> resolve_repo()
edk2_ci_setup.py	edk2_git_resolver.py	resolve_submodule_all()
edk2_setup.py	edk2_git_resolver.py	resolve_all() -> resolve_repo_all()

Main issues

• edk2_git.py Repo() is currently static. No values change once the class is created, regardless of if the physical repo changes. Additionally, it uses way more "RunCmd"s then necessary, as PyGit2 can quickly read the git database rather than run git commands themselves. edk2_git will be updated to rely on PyGit2 where possible to reduce the amount of RunCmd's used. As a side effect, it allows attributes to change depending on repo state (As an example, if say a file is updated, repo.dirty will be updated also).
• edk2_git_resolver.py (changed from repo_resolver) currently provides a few functions to ensure a repo meets the dependency requirements. I plan on adding new functions to do the same but based off the submodule requirements of the base repo. I.e. resolve() -> resolve_repo() and add a new resolve_submodule() function
• self_describing_environmept.py uses raw PyGit2 for discovering subtrees. This will simply be abstracted away behind edk2_git.py
• git_dependency.py only needs API changes based on edk2_git_resolver.py
• edk2_ci_setup.py only needs API changes based on edk2_git_resolver.py
• edk2_setup.py's core logic will be used as the basis for resolve_submodule(), using edk2_git.py Repo commands rather than RunCmd and edk2_setup.py will just need to call resolve_submodule() or resolve_submodule_all()

Overall, here is the current status of the project:

File`	Relies on	Updated?
edk2_git.py	N/A - Basis of repo functionality	Yes - #377
edk2_git_resolver.py	edk2_git.py	No
self_describing_environment.py	edk2_git.py	Yes - #377
git_dependency.py	edk2_git_resolver.py	No
edk2_ci_setup.py	edk2_git_resolver.py	No
edk2_setup.py	edk2_git_resolver.py	No