Add support for multiple Git push URLs

[jensens]

Summary

Adds support for Git's multiple push URL feature, enabling pushing to multiple remotes (e.g., GitHub + GitLab mirrors) automatically. Uses multiline configuration syntax consistent with version-overrides and ignores.

Motivation

Users often need to mirror repositories across multiple Git hosting services (GitHub, GitLab, Bitbucket) for redundancy, CI/CD pipelines, or organizational requirements. Git natively supports multiple push URLs per remote, and mxdev should support this workflow.

Changes

Core Implementation

1. Configuration Parsing (config.py):

• Added parse_multiline_list() function to parse multiline configuration values
• Modified package parsing to detect multiple pushurls and create pushurls list
• First pushurl stored in pushurl key for backward compatibility

2. Git Operations (vcs/git.py):

• Updated git_set_pushurl() to handle both single and multiple pushurls
• First pushurl set with git config remote.origin.pushurl URL
• Additional pushurls set with git config --add remote.origin.pushurl URL

3. Tests (tests/test_config.py, tests/test_git_additional.py):

• test_config_parse_multiple_pushurls(): Validates multiline parsing and backward compat
• test_git_set_pushurl_multiple(): Verifies git command sequence with --add flag
• test_git_checkout_with_multiple_pushurls(): End-to-end integration test

Configuration Example

[my-package]
url = https://github.com/org/repo.git
pushurl =
    git@github.com:org/repo.git
    git@gitlab.com:org/repo.git
    git@bitbucket.org:org/repo.git

When you run git push in the checked-out repository, Git will push to all configured pushurls sequentially.

Backward Compatibility ✅

Fully backward compatible:

• Single pushurl: pushurl = git@github.com:org/repo.git (unchanged behavior, no pushurls list)
• Multiple pushurls: New multiline syntax (creates pushurls list)
• Existing code checking "pushurl" in source continues to work
• Smart threading requires no changes (checks for presence, not count)

Testing

• ✅ All 197 tests pass (194 existing + 3 new)
• ✅ All linting checks pass (ruff, isort, mypy)
• ✅ Backward compatibility verified with existing single-pushurl tests
• ✅ New tests cover multiline parsing, git commands, and end-to-end integration

Documentation

• ✅ README.md: Updated pushurl table entry and added "Multiple Push URLs" section with examples
• ✅ CHANGES.md: Added feature entry under 5.0.2

Implementation Notes

Git Behavior:

• Git's native behavior pushes to ALL pushurls sequentially when git push is run
• If one pushurl fails, Git continues to the next and reports errors
• This implementation mirrors Git's native multi-pushurl support exactly

Design Decisions:

• Multiline syntax chosen for consistency with version-overrides and ignores
• ConfigParser doesn't support duplicate keys, so multiline is the natural choice
• Smart threading logic unchanged (checks "pushurl" in source for any value)

Related

Complements the smart threading enhancement from PR #69 - HTTPS URLs with any pushurl (single or multiple) are processed in parallel as they're assumed to be public/read-only.