Update to 2.12.0
    
    - Features
      * Add hub api --paginate option to automatically fetch all pages of results
      * Add hub pr show [<number>] command to open a pull request in the browser
      * Add hub pull-request --no-maintainer-edits flag to disallow project
        maintainers being able to push to your branch
      * Abort hub pull-request with a helpful error message if the current branch
        wasn't pushed to a git remote
      * hub issue -M now accepts milestone names instead of just numbers
      * Drop support for legacy hub.http-clone git config value
    
    - Fixes
      * Fix querying git configuration when GIT_TRACE is used
      * Support detached HEAD if hub pull-request --head was passed
      * Fix newline in hub create error message
      * Ensure HTTP connection reuse across API calls
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>