gerda: Better check that change was uploaded

Handle the case of merge commits -- they can't be found via gerrit
query, so we need to check if it is descendant of remote branch.
gerda: Refactor _fetch_changeset

Put argument parsing in separate function.
gerda: Minor code refactoring
gerda: Add open subcommand

New subcommand opens given change in browser. By default it uses
xdg-open, so your system defaults are used.