Despite the name, the --revs-only flag to git rev-parse doesn't make
it output only revision IDs. It makes it output only arguments that
are suitable for giving to git rev-list. So make start_rev_list and
updatecommits cope with arguments output by git rev-parse that aren't
revision IDs. This way we won't get an error when an argument such as
"-300" has been given to gitk and the view is updated.