Implement option --onto for subcommand 'new'

This option allows one to specify exactly where the new change-branch
should be created on.
Implement the global option --noop

This option is for "no operation". With it git-gerrit do not execute any
command that will change anything. This means, any Git command affecting
your working tree or any Gerrit REST call affecting the change status on
the server. Use it along with --debug to see what git-gerrit would do.
Substitute 'working tree' for 'working area' in the documentation
Create git_status routine to check working tree

This is just a refactoring to avoid duplication.
Update $Command global variable where needed