bash_completion: try to use bash3 features if they're available This does NOT break completion for bash 2.05b (I haven't tested earlier versions)

bash_completion: small cleanup and bugfix _hg_count_non_option wasn't taking global options into account while counting non-option arguments. It also used only its first argument.

teach bash_completion about --cwd

bash_completion: better handling of aliases Instead of listing every alias in the big case..esac, use hg help to get the canonical name of the command.