bug#24864: 26.0.50; bracketed paste regression

[Mark Oteiza <mvoteiza@udel.edu>]

Regression in master. I imagine this is a consequence of bug#24639 and
cf566b46

1. xterm
2. emacs -Q in said xterm
3. Shift-Insert a block of text into scratch buffer
4. C-/

I expect the entire paste operation to be considered a single insertion,
and for entire insertion to be undone.  However, only 20ish characters
at a time are being undone.  The behavior introduced by bracketed paste
is broken.

In the buffer, xterm-inhibit-bracketed-paste-mode is indeed nil.

In GNU Emacs 26.0.50.3 (x86_64-unknown-linux-gnu, X toolkit, Xaw scroll bars)
 of 2016-11-01 built on logos
Repository revision: c3640fcc96ed80368209c73d7ac9a0f0d1833d93

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --without-gconf --with-modules
 --with-x-toolkit=lucid 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe
 -fstack-protector-strong --param=ssp-buffer-size=4 -g
 -fvar-tracking-assignments -g -fvar-tracking-assignments'
 CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
LUCID X11 MODULES LIBSYSTEMD