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

[Durham Goode]

>>  
>>  the git --reuse-measage have a different semantic. we *NEED* another
>>name.
>>  
>>  --
>>  Pierre-Yves David
>  
>  What about "--keepmessage"?
>  Angel

--keep-message was what I had in mind originally, but others thought using
git's flag might make it easier for git refugees.  I don't think the
slightly different semantic means we can't use the same name.

Both names are descriptive enough and both are long, so I don't think
there's any real difference.

-Durham