cli: Force d2 fmt operate when the input file is stdin

[akirak]

This PR fixes #1577.

The change is obvious. Previously, d2 fmt CLI command was no-op when the content is unchanged. However, this check should be skipped if the input file is stdin–because editor integrations would use it for on-the-fly reformatting.

[alixander]

Thank you for the PR but I don't think stdin should be treated any differently.

#1577 (comment)

Following what gofmt would do, d2 fmt ... should output to stdout and d2 fmt -w ... should overwrite the files.

[akirak]

I don't think stdin should be treated any differently.

We should just conform to what https://pkg.go.dev/cmd/gofmt does.

Is this implemented even now? From the document on gofmt, you see the following description:

Without an explicit path, it processes the standard input.

but d2 fmt requires at least one file argument, as seen in the below example:

01:40 $ d2 fmt < sample.d2
err: failed to fmt: bad usage: fmt must be passed at least one file to be formatted
err: Run with --help to see usage.

The definition of conformance looks vague here.

[akirak]

Okay, I think d2 fmt should first support zero arguments. Closing this PR anyway.