Add CommentStoreComment::newUnsavedComment()

Add CommentStoreComment::newUnsavedComment()

Revision (or more accurately, its upcoming MCR replacement) should allow
for the full flexibility of CommentStore in its interface, and the most
straightforward way to do that is to use CommentStoreComment. But for
that to work sanely, we need a non-@private way to create a new
CommentStoreComment that hasn't been saved to the database yet.

Change-Id: I76028e8ef3da4568a16e098cdfbc806a67b814e3