branch-status: allow more chars in branch name

This probably introduces a bug with right square bracket (]) in the
first line of the newest commit message. But I don't care because:

1) I couldn't figure out how to add right bracket as part of the
   character exclusion; i.e., none of the following worked:

   * [^ \]]
   * [^ \\\]]
   * [^ ]]
   * [^ \\]]

   (attempted in that order)

2) Use of a right bracket in a commit subject is very rare.