Suppress updating page excerpt when rendering in the preview tab

When composing a draft and previewing a page with an excerpt, the excerpt for the page will be set to the excerpt in the draft even if the draft is never saved. Any pages that include the excerpt using excerpt-include will see the updated excerpt until the excerpt is reset by viewing the original page in the pre draft state.

Effectively the preview tab can result in release of page content which is against the users' expectation that their modifications will not be visible until the page is saved.

To prevent this the excerpt macro should suppress updating the page excerpt when rendering for the preview tab.