Add Git commit diff source for tracking code changes

[butschster]

This PR introduces a Git integration feature that allows tracking and including code changes from Git commits in context files. The new functionality enables:

• Including changes from specific commits, commit ranges, or time periods
• Filtering diffs by file patterns, paths, and content
• Rich preset aliases for common use cases ('last', 'today', 'last-week', etc.)
• Support for branch comparison patterns ('main-diff', 'master-diff')
• Visual tree representation of changed files
• Optional inclusion of Git stats with diffs

Example usage in configuration:

{
  "type": "git_diff",
  "repository": "./",
  "description": "Recent Git Changes",
  "commit": "last-week", 
  "filePattern": "*.php",
  "notPath": ["tests", "vendor"],
  "showStats": true
}

Example document

recent-changes.md