shells/bosh: Update to 20200701

Take maintainership.

Changelog from AN-2020-07-01:
- Bourne Shell: A command like

      bosh -c 'var=$({ echo value >&3; } 3>&1); echo $var'

  Did not print "value" as expected, but rather bosh: 3: bad file number

  This is related to the fact that curly brackets do not have an
  own binary node type in the parsed output and the recursive parser
  from the $(cmd) execution needs to reconstruct the text form
  of a command tree. As a result, the command has been translated
  into:

      bosh -c 'var=$( echo value >&3 3>&1); echo $var'

  and this caused the observed problem.

  Note that this basic problem may not be finally fixed, so please
  report in case that other commands with curly brackets together
  with $(cmd) cause problems.

  Thanks to Koichi Nakashima for reporting.

- Bourne Shell: Added a unit test for the bug above.

(micha)