Issue Summary

When running git 2.48, merging a pull request from a branch in a fork to the upstream branch changes the default branch in both the fork and the upstream. 

Steps to Reproduce

1. Fork repository R to create a personal repository R1.
2. R1 contains two branches: R1/A (default) and R1/B, mirroring the original repository.
3. Implement changes in the R1/B branch.
4. Create a pull request to merge changes from R1/B into R/B.
5. Merge the pull request.

Expected Results

The default branch is not changed on either repo

Actual Results

After merging, both R and R1 have branch B set as the default branch.

Workaround

Downgrade to another supported git version other than 2.48.x