bug#30213: 26.0.91; shell buffer not displayed when not erased and async-shell-command-display-buffer is used

[Alex Branham]

With async-shell-command-display-buffer set to t and
shell-command-dont-erase-buffer set to 'beg-last-output, async shell
buffers do not get displayed after the second async call, presumably
because the buffer doesn't get erased so there isn't a first line
written.


In GNU Emacs 26.0.91 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.26)
 of 2018-01-22 built on mars
Repository revision: 1fc98ed073a2d26ad941748ecb13aa6f47dd7b01
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --mandir=/usr/share/man --with-mailutils
 --with-gameuser=:games --with-sound=alsa --with-modules --without-gconf
 --without-gsettings --without-gpm --without-m17n-flt
 --without-imagemagick --with-xml2 '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
 -fuse-ld=gold''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND DBUS NOTIFY ACL GNUTLS LIBXML2 FREETYPE
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