cherry menu.skip branch enhancement

[jpsaman]

Blacklisting a commit for a specific branch by hand on each git notes edit is cumbersome when working on a large patch set. The addition of an option to add a specific branch instead of 'all' improves the git-cherry-menu work flow.

To ease the workflow git-cherry-menu gained an option to override the default blacklist branch 'skip: all' by specifying the option '-c cherry-menu.skip-branch='.