Amending a signed commit overwrites author

### Describe the bug

Amending a signed commit overwrites the author (bad) and the committer (good).

Amending an unsigned commit does not overwrite the author (good), but it also does not overwrite the committer (bad).

### To Reproduce

1. Check out a branch where the latest commit was authored and committed by someone else
2. Run `git log --pretty=full -n 1` and verify that the `Author` field shows someone other than you
3. Stage changes
4. Open the commit dialog
5. Press Ctrl-A to make the pending commit be an amend operation
6. Press Ctrl-D to finish the commit
7. Run `git log --pretty=full -n 1` again
8. Observe that either the author or committer is wrong (depending on whether the commit you amended was signed)

### Expected behavior

Amending a commit should overwrite the committer, but not the author.

### Context

- OS: Chimera Linux (updated as of 2025-11-20)
- GitUI Version: 0.27.0
- Rust version: 1.91.0