why: Document the pytest plugin fix for users upgrading
what:
- Add changelog entry for git_repo fixture set_home dependency

why: The fixture was returning early if .gitconfig already existed,
even if the file was empty or missing protocol.file.allow=always.
This caused failures on Arch Linux build where git/container setup
creates an incomplete .gitconfig before the fixture runs.

what:
- Remove early return that skipped writing when file exists
- Fixture now always writes the full config with protocol.file.allow
- Follow-up to #510, #511 for #509

why: The fixture was returning early if .gitconfig already existed,
even if the file was empty or missing protocol.file.allow=always.
This caused failures on Arch Linux build where git/container setup
creates an incomplete .gitconfig before the fixture runs.

what:
- Remove early return that skipped writing when file exists
- Fixture now always writes the full config with protocol.file.allow
- Follow-up to #510, #511 for #509

## Summary

- Fix `gitconfig` fixture to always write config - removes early return that skipped writing when file exists
- Follow-up to #510, #511 for #509

## Problem

The `gitconfig` fixture had an early return at lines 157-158:

```python
if gitconfig.exists():
    return gitconfig
```

If ANYTHING creates `.gitconfig` first (empty or incomplete) - git itself during `git init`, the nspawn container setup, another fixture - the fixture returns early WITHOUT writing `protocol.file.allow=always`.

Confirmed by reporter running debug script: empty `.gitconfig` was created by container/git before the fixture could write the proper config.

## Solution

Remove the early return. The fixture now always writes the complete config with `protocol.file.allow=always`.