Preserve isNew after save completes successfully

[aduth]

See #907 (comment)

This pull request resolves an issue where the state.saving.isNew value is inaccurate after a post is saved successfully.

Testing instructions:

Repeat instructions from #907 (comment)

Ensure unit tests pass:

npm test

[nylen]

Missed a spot: https://github.com/WordPress/gutenberg/pull/907/files#diff-aad59fa5eb910a7034e7a3cd9b491895L66

As far as the user experience, this controls the brief flash of text that appears when a save request is in progress. Prior to this PR it will say "Updating..." even for a new post; fixed in 2fec04e.

[nylen]

As a separate task, I think the confusion here indicates that this part of the state tree should be restructured a bit.

[aduth]

Thanks @nylen for noticing, explaining, and fixing 👍 Will merge shortly.

[nylen]

@aduth, sorry, I meant to say something sooner, but #945 removes this part of the state tree entirely. So this PR may not be needed anymore.

[aduth]

My fault for being slow on revisiting my own pull requests. I'll plan to catch up with the activity there.

[aduth]

Closing in favor of #945