bug#34192: 26.1; Bash 5.0 breaks TRAMP sudo

[Allen Li <darkfeline@felesatra.moe>]

I believe Bash 5.0 breaks TRAMP sudo in
tramp-sh-handle-file-name-all-completions.

Using Bash 5.0 and (find-file "/sudo::/") results in an error "Process
has died".  Setting tramp-verbose to 9 shows that it dies around here:

13:56:22.449785 tramp-maybe-send-script (5) # Sending script
‘tramp_perl_file_name_all_completions’...
13:56:22.449920 tramp-send-command (6) #
tramp_perl_file_name_all_completions () {
\perl -e '
opendir(d, $ARGV[0]) || die("$ARGV[0]: $!\nfail\n");
@files = readdir(d); closedir(d);
foreach $f (@files) {
 if (-d "$ARGV[0]/$f") {
  print "$f/\n";
 }
 else {
  print "$f\n";
 }
}
print "ok\n"
' "$1" 2>/dev/null
} 2>/dev/null; echo tramp_exit_status $?
13:56:25.428280 tramp-maybe-send-script (5) # Sending script
‘tramp_perl_file_name_all_completions’...failed

Note that I cannot reproduce this from emacs -Q because out of the box,
TRAMP sudo does not seem to call
tramp-sh-handle-file-name-all-completions and I don't have time right
now to figure out what setting enables that code path.

Switching my /bin/sh to Dash instead of Bash 5.0 immediately fixed the
problem, which strongly led me to suspect some subtle change in Bash 5.0
broke this.

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 11.0.12003000

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