Bumping part required by another ignores the requiring part

[markmacode]

Example config

parts:
  major:
    value: 3
    requires: minor

  minor:
    value: 9
    prefix: '.'
    requires: patch

  patch:
    value: 2
    prefix: '.'

  pre:
    value: null
    prefix: '-'
    requires: prenum
    identifier:
      strings: [ 'alpha', 'beta', 'rc' ]

  prenum:
    prefix: '.'
    value: null
    number:
      start: 1

Problem

The above config will give a version of 3.9.2, and if we do myver --bump prenum then we will get a version of 3.9.2.1.

The pre part will remain as null if you only bump prenum. We should instead expect to get a version of 3.9.2-alpha.1 because the pre part requires the prenum part, and they should always exist together.