Dvc: scripts: Zsh completion should work with subcommands

[ghost]

For example: dvc metrics show <tab>.

The way to support this would be to modify the way we parse our arguments to support subcommands https://github.com/iterative/dvc/blob/dd8598e53ea03aaa7554c6ebf8fa7fe37baffc31/scripts/completion/dvc.zsh#L231-L240

[jorgeorpinel]

I just want to add that specifically for dvc cache dir not all the $_dvc_global_options exist (-q, and -v specifically), while for all the other subcommands I think they do. Something to consider. (Maybe they should be added to the subcommand?)

[efiop]

@jorgeorpinel Good catch! Indeed, we've missed those there. Looks like we just need to use parents=[parent_parser, parent_config_parser] in https://github.com/iterative/dvc/blob/master/dvc/command/cache.py#L46 .

[casperdcl]

also see https://stackoverflow.com/questions/9000698/completion-when-program-has-sub-commands

[efiop]

Fixed in https://github.com/iterative/shtab