completion: autogenerate (#3921)

* cli: split main_parser

* cli: basic parser tree printing

* cli: major completion tidy

* completion: more missing root commands

* completion: much automagic such beauty

- fixes #2985 automatically generate bash completion

* tests: bash completion

* tests: completion: tidy script a little

* minor tidy

* command: positional name consistency

* completion: bash: add logging

* double-check renames and consistency

* cli: add completion subcommand

* ci: completion: update test

* minor logging

* more completion logging verbosity

* fix completion of files

* add shtab

* purge unneeded completion => shtab

* completion: neaten UI

* completion: use `shtab>=0.0.2`

* completion: purge hardcoded completions!

* revert unneeded renames for now

* fix isort

* completion: tidy CLI API

* fix snap build

* fix snap build again

* snap: internally build completion

* allow repo-less completion

* command: tidy headless logic

* use CmdBaseNoRepo

* better DVC-file completion

* merge command.choices -> command.completion

* drop choices class

* remove completion optionals

* tidy docs

dvc/command/commit.py

@@ -1,6 +1,7 @@
 import argparse
 import logging
 
+from dvc.command import completion
 from dvc.command.base import CmdBase, append_doc_link
 from dvc.exceptions import DvcException
 
@@ -66,5 +67,6 @@ def add_parser(subparsers, parent_parser):
         nargs="*",
         help="DVC-files to commit. Optional. "
         "(Finds all DVC-files in the workspace by default.)",
+        choices=completion.Optional.DVC_FILE,
     )
     commit_parser.set_defaults(func=CmdCommit)