bug#64185: proposal for new function: copy-line

[Juri Linkov]

>> > We can certainly do better.  I just pushed another 10× improvement, on
>> > top of the previous performance doubling for duplicate-line.
>>
>> Thanks, this is a nice improvement.  Too bad you did this on master.
>> This means I can't push the patch that Eli approved to emacs-29 anymore,
>> because whoever will merge emacs-29 to master next time will be forced
>> to resolve the merge conflict, and this is not a trivial task.
>
> You can install on emacs-29 with a commit log message which says
> "don't merge to master".  Then Someone™ will have to do the equivalent
> fix on master.

This means more work.  Ok, done.

I don't know if this can be closed now, since a new option for
duplicate-dwim could be created here as well.

Then e.g. duplicate-dwim-final-position could inherit its default value
from duplicate-line-final-position.  And here more general values 0, 1, -1
become an advantage since inherited symbol values such as 'last-new-line'
would not make sense for the region part of duplicate-dwim.