bug#21119: comment-dwim should have behavior on active, empty regions

[Zachary Kanfer <zkanfer@gmail.com>]

This patch is for the function comment-dwim, which is bound to C-; by default.

Currently, when the region is not active, comment-dwim puts a comment
at the end of the line. When the region is active, comment-dwim
comments out the region by calling comment-or-uncomment-region.
However, when the region is active, but empty,
comment-or-uncomment-region does nothing; no text is inserted. I think
this last action (upon active, empty region) is a bug. If the active
region is empty, we should insert a comment at the end of the line, as
we do when there's no region.

This patch calls use-region-p to detect whether the region is active,
rather than checking mark-active transient-mark-mode explicitly. The
effect of this is, when the region is active, but empty, comment-dwim
now acts as though the region was not active -- it puts a comment at
the end of the line. Use-region-p does do the previous behavior (of
checking mark-active and transient-mark-mode) by calling
region-active-p.

The patch follows:

From 6bdd96b07775c5c2fd0f3d6595b6655b4093dcf2 Mon Sep 17 00:00:00 2001
From: Zachary Kanfer <zkanfer@gmail.com>
Date: Mon, 20 Jul 2015 01:36:56 -0400
Subject: [PATCH] Make comment-dwim treat empty regions as inactive.

When the region is active, but is empty (length 0), comment-dwim now
acts as though the region was not active; that is, it puts a comment
at the end of the line.
---
 lisp/newcomment.el | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lisp/newcomment.el b/lisp/newcomment.el
index 172a563..8d17a69 100644
--- a/lisp/newcomment.el
+++ b/lisp/newcomment.el
@@ -1266,7 +1266,7 @@ Else, call `comment-indent'.
 You can configure `comment-style' to change the way regions are commented."
   (interactive "*P")
   (comment-normalize-vars)
-  (if (and mark-active transient-mark-mode)
+  (if (use-region-p)
       (comment-or-uncomment-region (region-beginning) (region-end) arg)
     (if (save-excursion (beginning-of-line) (not (looking-at "\\s-*$")))
     ;; FIXME: If there's no comment to kill on this line and ARG is
--
1.9.1