chore: Adding initial git function mocks

Description

As part of the discussion at !1945 (comment 2391909469), this MR introduces a git_mocks package make glab testing and git features easier to add.

The plan will be to:

• introduce a mock-friendly version of all the functions in the normal git package
• transition glab commands that use the git package to use the git_mock package
• make sure there are adequate tests for each function and command that uses the function
• remove the old git package

Related Issues

Resolves #[issue_number]

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