fix initial origin of editor box

[Ryooooooga]

• PR Description

When trying to reword a commit with a multi-line message, only the last line is displayed in the edit box.

To reproduce

1. commit changes with multi-line message: git commit -m "$(seq 100)" --allow-empty

2. launch lazygit

3. move to Commits panel (3), reword latest commit (r)

before

after

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go run scripts/cheatsheet/main.go generate)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs (specifically docs/Config.md) have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[jesseduffield]

awesome work!