bug#21921: New function comment-line also comments first line after region

[Clément Pit--Claudel]

Hi all,

The new comment-line feature is very convenient, but I find its operation 
slightly confusing wrt the "comment out region" feature.

Its documentation says:

  If region is active, comment lines in active region instead.

However, it seems that it also comments out the line just after the end
of the region.

For example, given the following text

abc
def
ghi

Placing the cursor before `a', then pressing C-SPC C-n C-n, causes two
lines to be marked: (> is highlighting, | is point)

>abc
>def
|ghi

Yet pressing C-x C-; (comment-region) comments out all three lines.

Similarly, again starting from 

abc
def
ghi

placing the point before `g' and pressing C-SPC C-p C-p causes two lines
to be marked:

>|abc
>def
ghi

Yet pressing C-x C-; comments out all three lines. I find this second example 
especially confusing.

Is this behaviour only surprising to me?

Cheers,
Clément.

In GNU Emacs 25.1.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2015-11-14
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