Commit Staged (Amend) asks for new commit message

[chrmarti]

• VSCode Version: Code - Insiders 1.17.0-insider (a5210bc, 2017-09-26T05:24:48.911Z)
• OS Version: Windows_NT x64 10.0.15063
• Extensions: none

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Testing #34963

Steps to Reproduce:

1. Stage some change
2. Commit Staged (Amend)
3. 🐛 I'm asked for a new commit message without the existing one as the default. I usually want to keep the existing one.

[pradeepmurugesan]

@joaomoreno yeah may be we can add an option to keep the older message. I mean prefill the text box with the latest commit message..