Fix Git tag update regression

[jensens]

Summary

Fixes #46: Git tags in branch option are now correctly handled during updates

This PR fixes a regression introduced in v3.x/v4.x where updating from one Git tag to another stopped working. Tags were incorrectly treated as branches during the update process.

Current Status: Demonstrating the Bug

Failing Test

The first commit adds test_update_git_tag_to_new_tag() that demonstrates the issue:

Test scenario:

1. Create repo with tag 1.0.0
2. Initial checkout with branch = 1.0.0 (works via git clone -b)
3. Update config to branch = 2.0.0
4. Run update (FAILS)

Error:

SystemExit: 1
src/mxdev/vcs/git.py:226: in git_switch_branch
    self.output((logger.error, f"No such branch {branch}"))
    sys.exit(1)

Root Cause

In git_update() at lines 249-251:

elif "branch" in self.source:
    stdout, stderr = self.git_switch_branch(stdout, stderr)
    stdout, stderr = self.git_merge_rbranch(stdout, stderr)

This code assumes the value in branch is an actual Git branch. When it's a tag:

1. git fetch runs successfully (fetches tags)
2. git_switch_branch() looks for the tag in git branch -a output
3. Doesn't find it (tags ≠ branches)
4. Exits with error: "No such branch 2.0.0"

Why initial checkout works: git clone -b 8.0.0 recognizes tags and works
Why update fails: The update logic only looks in branch listings

Next Steps

• Implement tag detection in git_update()
• Add logic to handle tags separately from branches
• Verify test passes
• Run linting
• Update CHANGES.md
• Mark PR ready when CI is green

Test Plan

# Currently fails:
pytest tests/test_git.py::test_update_git_tag_to_new_tag -v

# After fix, should pass