bug#32791: 26.1; rgrep uses null byte instead of colon separator

[Allen Li <darkfeline@felesatra.moe>]

Reproduce:

1. mkdir -p /tmp/tmp
2. echo foo >/tmp/tmp/tmp

1. emacs -Q
2. M-x rgrep RET foo RET * RET /tmp/tmp RET
3. Move point onto the first colon in ./tmp:1:foo
   and press C-u C-x =
3. Move point onto the first colon in ./tmp:1:foo
   and press C-u C-x =

For the commands that actually run grep, like M-x grep, both of these
colons are actually colons.  For rgrep (and I would guess the other
commands implemented in Emacs Lisp), the first colon is a NUL byte
displayed as a colon.

This breaks wgrep.el for rgrep, which parses for the two colons in each
line.

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-07-05 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12001000

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

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
GTK3 X11 MODULES THREADS LIBSYSTEMD LCMS2

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix