completion: misc fixes (#4076)

* command: add metavars

- makes help output look the same as before #3921

* deps: update shtab version

Part of #4070

dvc/command/commit.py

@@ -67,6 +67,7 @@ def add_parser(subparsers, parent_parser):
         nargs="*",
         help="DVC-files to commit. Optional. "
         "(Finds all DVC-files in the workspace by default.)",
+        metavar="targets",
         choices=completion.Optional.DVC_FILE,
     )
     commit_parser.set_defaults(func=CmdCommit)