Allow 'git gerrit new' while on a change-branch

If we're already on a change-branch the new change-branch is based on
the current one's upstream branch.
Document how to comment on a new patchset with push && review
Make 'show' accept a list of changes
Make 'checkout' accept a list of changes

All changes are fetched and local change-branches created for
them. The last one is checked out.
Refactor: new routine grok_unspecified_change