Add support for start-point in checkout step

[jovnc]

Description

To support git-mastery/exercises#65 and future exercises, this PR adds the ability to create a new branch from a specific starting point in the checkout step. This enables scenarios like creating a branch from a previous commit (e.g., git checkout -b new_branch HEAD~1).

Relevant PR: git-mastery/exercises#125

Changes Made

1. New start-point Field in Checkout Step

• Field: start-point (optional, string)
• Behavior: When provided with branch-name, creates a new branch at the specified commit reference
• Git Command Equivalent: git checkout -b
• Must be specified together with branch-name and given branch-name cannot exist (we are not checking out to a branch/commit, we are creating a new branch from specified starting point)

2. Supported start-point Formats

• Relative commit references: HEAD~1, HEAD~2
• Commit SHAs: abc1234 (can be full or abbreviated)
• Branch names: main, feature/xyz
• Tags: v1.0.0

3. Add tests

• unit test
• integration test

4. Add to details to specification.md

[jovnc]

@woojiahao Opened this PR to help with git-mastery/exercises#125