Show GPG error before entering commit editor when rewording non-latest commits

[m04f]

• PR Description

When GPG signing is enabled but overrideGpg is false, attempting to reword a non-latest commit would show the "Feature not vailable for users using GPG" error only after the user had already entered the commit message.

Now the error is displayed immediately when attempting to start the reword operation, before opening the editor.

Closes #4611

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
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• Docs have been updated if necessary
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