heredoc whose eof-mark has backquotes becomes quoted

[Denys Vlasenko]

Background:

Unquoted heredocs are this construct
cat <<EOF
heredoc will expand $var, `cmd` etc
EOF

Quoted heredocs are this construct:
cat <<"EOF"
these chars are literals: $ `
EOF

"Quoteness" also can be achieved by bkslash
or single quotes: EO\F 'EOF' EOF"" ''EOF


The bug: if there are backquotes in eof-mark,
they become the part of eof-makr (this is ok), but they also
make this heredoc _quoted_, which should not happen:

cat <<EO`true`F
heredoc1
EO`false`F
EO`true`F
echo Ok:$?

Output of bash-4.3.43:

$ bash z
heredoc1
EO`false`F
Ok:0

Should be:

heredoc1
EOF
Ok:0