bug#23685: Emacs 25.0.94.1; Undo is clumping together delete-forward-char commands from distant parts of the buffer.

[Alan Mackenzie]

Hello, Emacs.

I'm using Emacs 25.0.94.1, built as follows:

In GNU Emacs 25.0.94.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.18.7)
 of 2016-05-24 built on acm
Repository revision: 2671179b11a9423792aaf2439a26a2562679c086
Configured using:
 'configure --with-tiff=no --with-gif=no --with-gpm'

Configured features:
XPM JPEG PNG RSVG SOUND GPM DBUS GSETTINGS NOTIFY ACL GNUTLS LIBXML2
FREETYPE XFT ZLIB TOOLKIT_SCROLL_BARS GTK3 X11

I have the following in a buffer:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
#define FOO(bar) char bar [] = R"bar(bar)bar"

char baz [] = R"baz(baz)foo";
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

I delete the "a" from Line 1 col 34 with `delete-forward-char' (bound to
the <deletechar> key).  Then I move to Line 3, and likewise delete the
"o" in column 25.

Now I attempt to undo the last deletion with C-_.  Maddeningly, this
undoes both of the two last disjoint deletions.  It appears to be
impossible to undo only the last of these deletions.  This has caused me
some inconvenience in testing.  There is no customizable option by which
this irritating behaviour can be disabled.  It is surely a bug.

I can understand coalescing a sequence of undos in the same place, much
as consecutive `self-insert-command's are coalesced into a single group
for undo.  However, coalescing two deletions separated by movement
commands into a single undo group is going too far.

Can we possibly have a rethink on this before Emacs 25 goes out?

-- 
Alan Mackenzie (Nuremberg, Germany).