[PATCH RFC] commit: add --reuse-message for keeping the old commit message during amend

[Durham Goode]

>We usually don't use '-' in option names.
>

--close-branch is an option on commit already, so there's some precedence.

>
>It would perhaps be better to name the option after what it does instead
>of what it doesn't do. Perhaps something like 'filesonly'.

I think --reuse-message is more intuitive.  In some way it does describe
what the commit does: it reuses the message.  And the name more closely
aligns with the user's intent.

>
>This option seems to be the opposite of 'qref -e' ... but I guess it is
>too late to change the default for amend ...

This would be ideal, but alas...