completion: bash: allow multi-arg completion, minor tidy

[casperdcl]

• allow subcommand bash completion for more than one arg
  • e.g. dvc metrics add would only complete one more argument before
• misc minor tidy
• related: Fix bash completion #3360 <- Autocomplete for file names doesn't work (partially) [bash] #1923

[efiop]

Thanks @casperdcl !