chore: Add additional mocks to git package

Description

As part of Convert the `git` package into functions that c... (#7896), adding (more) mocks to the git_mock

This adds:

• CheckoutNewBranch(branch string) error
• UncommittedChangeCount() (int, error)
• GitUserName() (string, error)
• LatestCommit(ref string) (*Commit, error)
• Commits(baseRef, headRef string) ([]*Commit, error)
• CommitBody(sha string) (string, error)
• Push(remote string, ref string) error
• HasLocalBranch(branch string) (bool, error)

And makes a small change to:

• RemoteBranchExists(remote string, branch string) (bool, error)

How has this been tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap