Subcommand parsing: "fooasdf -o=foo" is incorrectly identified as the subcommand "foo"

[kmoe]

Consider a command with a single subcommand foo.

Running the command with arguments fooasdf -o=bar returns a usage error, return code 127.

However, running the command with arguments fooasdf -o=foo does not return an error, and instead sets the subcommand to foo. This is incorrect - this example should exit 127 just as above.

This problem was seen in hashicorp/terraform#30870, in which terraform planasdf -out tfplan, which should error, instead behaves as if the user had run terraform plan -out tfplan.

The problem seems to be that the subcommand matching logic at https://github.com/mitchellh/cli/blob/master/cli.go#L704 is too liberal, and identifies any string both prefixed and suffixed by a subcommand as that subcommand.