Improve Git process management and repository discovery performance

[eamodio]

Summary

Improves system responsiveness by adding intelligent Git process management and optimizing repository discovery.

Impact

• Adds a priority-based concurrent queue to prevent Git operations from overwhelming system resources
• Implements three priority levels: interactive (immediate), normal, and background (throttled)
• Adds gitlens.advanced.git.maxConcurrentProcesses setting to tune concurrent Git process limits
• Combines rev-parse calls to reduce Git process spawning during repository discovery
• Staggers repository resumption on window focus to maintain UI responsiveness

Validation

• Test with large repositories and many open files
• Verify interactive Git operations remain responsive during background tasks
• Check repository discovery time on workspace open
• Test window focus with multiple repositories

Risk

Medium - Core Git execution pipeline changes. New setting added for user tuning.

[eamodio]

Closed by 872e67c, 0bac8ea, and 5adea75