Fix credential prompts overlapping in parallel VCS operations

[jensens]

Summary

Fixes #35: Prevents credential prompts from overlapping when using HTTPS URLs in parallel VCS operations.

Problem

When mxdev processes multiple HTTPS packages in parallel (default: 4 threads), git credential prompts get mixed with output from other threads, making it confusing for users to enter credentials.

Solution: Smart Threading

Added a new configuration option (enabled by default) that uses a two-phase approach:

Phase 1: Process HTTPS packages serially (threads=1)

• Clean, visible credential prompts
• Git caches credentials after first prompt

Phase 2: Process remaining packages in parallel (threads=N)

• SSH and local packages processed at full speed
• No credential prompts expected

Configuration

[settings]
# Enable intelligent threading (default: true)
smart-threading = true

Set to false if you have credential helpers configured and never see prompts.

Implementation Details

• Applied to both checkout() and update() methods
• Automatically detects HTTPS URLs (url.startswith('https://'))
• No new dependencies - pure stdlib
• Backward compatible - no config changes required

Changes

• config.py: Added smart-threading setting with default true
• vcs/common.py:
  • Added smart_threading parameter to WorkingCopies.__init__()
  • Added _separate_https_packages() helper method
  • Refactored checkout() to use two-phase processing
  • Refactored update() to use two-phase processing
• processing.py: Pass smart_threading to WorkingCopies
• CHANGES.md: Documented new feature

Testing

• ✅ All existing tests pass
• ✅ Linting passes
• Manual testing needed for credential prompt scenarios

Test Plan

# Test with HTTPS packages requiring credentials
# Verify prompts are clean and sequential
pytest tests/test_git.py -v

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cc @jensens