bug#17757: 24.4.50; Whitespace changes break signature mark

[Stefan Monnier]

>> C-x m (compose-mail)
>> M-> (end-of-buffer)
>> "-- " (i.e., dash - dash - space)
>> RET
>>
>> Now the space after the sigdashes disappears.
>
> Tracing this a bit more, it seems that this is what's deleting the space
> after the two dashes:

I blame that old sig convention: trailing whitespace has always been
a source of trouble.

E.g. I can see that we might want to preserve the space in the above
case (tho it presumes that the user really intended it to be the sig
separator, which seems quite unlikely nowadays because AFAICT most
users have no idea that such a thing even exists).
But what about:

    M-> -- SPC hello world M-b M-b RET

should this RET be interpreted as an intention to turn the "-- SPC" into
a sig separator?


        Stefan