Expanded commit message editor doesn't update when amend check box is toggled

[kuychaco]

1. Ensure amend box is unchecked and type "Original message"
2. Check amend box
3. Expand commit box to open editor
4. Uncheck amend box (note that commit message in editor does not update to show "Original message")
5. Close commit message editor
6. Click amend check box and note that original commit message is not restored (potential data loss issue if people want to recover "Original message"

Expected: Not sure. To be determined, 💭 @iolsen @ungb @simurai
Actual: Message in editor does not update

Ref: #1211

[kuychaco]

Punting on this until proper UX is fleshed out and focusing on higher priority items.

Some initial thoughts and options discussed with @BinaryMuse for posterity:

1. disable the amend check box when the editor is expanded, forcing the user to close the expanded editor in order to toggle amending state
2. updating the editor buffer when checking/unchecking the amend button (as is the case in the commit message box) - this might be jarring/unexpected, or even undesired if the user wants to retain their commit message and use it to amend their last commit.
3. prompting the user before updating the editor buffer, giving them the option to keep their commit message, or replace it with the message from their last commit

Things to take into consideration for 2 and 3 - when do you update the in-memory messages (regular and amending message) as well as the on-disk message?