bug#15746: Fix committed to master.

[Karl Fogel]

I've pushed a fix to master for this (commit 3812e17978).

Essentially it takes Drew's suggestion.  I left "C-x r m" as `bookmark-set', 
with the same behavior it has now, and "C-x r M" sets a bookmark but errors if 
it would overwrite an existing bookmark of the same name.

I didn't go with Stefan's suggestion of using RET RET instead of RET, because 
you can't habituate to it.  You don't necessarily know in advance whether 
you're setting a new bookmark or updating an existing bookmark, so you can't 
predict whether you will need to hit RET or RET RET.  Better to offer two 
different commands, and those people who want one style all the time can just 
use the appropriate command all the time.

Comments and further discussion welcome, of course.  Thanks, Leo, for raising 
this issue, and thanks Drew and Stefan for helping think things through in the 
bug ticket.