Add branch push support to PR creation workflow

[csvenke]

Summary

Implements automatic branch pushing when creating pull requests. If a branch doesn't have an upstream, it will be automatically pushed to the origin before PR creation.

Key Changes

• Client Interface: Added HasUpstream() and PushWithUpstream() methods to the gh.Client interface
• Git Operations: Implemented upstream checking and branch pushing in RealClient
• PR Command: Modified to fetch current branch and pass it to the pull request creation function
• PR Creation Logic:
  • Created new CreatePullRequestWithBranch() function that handles upstream checking and branch pushing
  • Maintained backward compatibility by keeping original CreatePullRequest() function
  • Updated function signature to accept branch name and git client parameters
• Tests: Added comprehensive test coverage for the new branch push functionality including error cases

Implementation Details

• Branch pushing only occurs when both branch name and git client are provided
• Gracefully handles cases where upstream already exists (skips unnecessary push)
• Proper error handling for upstream checking and push operations