It seems that under python3, s3cmd emits its output as a long list (like
ls -l) with or without the --long parameter to "s3cmd ls s3://...".
This patch includes only s3://* paths from that output as completions.
Per the discussion with @faho in #4332, replaced some custom completion
state detection functions with standard __fish_* functions used in other
completion sources.
No longer auto-generated. Everything has been summarized. Supressing
file completions for initial command, providing list of valid initial
commands, filtering --options by subcommand.
