Signed-off-by: Gabriel Smith <ga29smith@gmail.com>

This "command" argument allows rustup to output the completion information of
more than just rustup. In this case it adds a cargo completion script.

Signed-off-by: Gabriel Smith <ga29smith@gmail.com>

This allows the cargo script for the current default toolchain to be called,
and if the script is ever updated the user won't have to maually update their
completion script after updating Rust.

Signed-off-by: Gabriel Smith <ga29smith@gmail.com>

Signed-off-by: Gabriel Smith <ga29smith@gmail.com>

This removes the duplication between the `variants` and `from_str`
implementations.

Moves the error reporting from the completion function to the standard
error handling mechanism.

CompletionCommand had to be made fully public, not that this matters for
a binary target. That said, this could have been prevented by
transforming the CompletionCommand into its String form at the error
site rather than at the reporting site. It was chosen to go with this
method as stringly-typed interfaces are generally bad.

Compares two different command invocations and panics if their return
code and stdout/stderr don't match.