Only patch workspace only calls to plots diff

[mattseddon]

Address one of the points from #2581

Plots update command runs with extra arguments for no reason.

From #2581 (comment):

The last item can be narrowed down to this:

vscode-dvc/extension/src/plots/model/index.ts

Lines 204 to 206 in c83e763

	public getDefaultRevs() {
	return ['workspace', ...Object.values(this.branchRevisions)]
	}

and this PR:

https://github.com/iterative/vscode-dvc/pull/1949/files

One the issue is fixed now.

Btw, using the workspace revision alone gives me an error (I would expect this, since workspace is not an actual Git revision):

plots,diff,workspace,-o,.dvc/tmp/plots,--split,--json failed with ERROR: unknown Git revision 'workspace' retrying...

I am reluctant to unwind any of the changes made in #1949 right now and they are unrelated to this particular change.

In order to get significant performance increases we will need to re-do this sub-system and we will need help from the CLI.

[julieg18]

Looks good to me!

[codeclimate]

Code Climate has analyzed commit e3d1d35 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (85% is the threshold).

This pull request will bring the total coverage in the repository to 96.8% (0.0% change).

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