Summarizing a topic makes previous history invisible

Submitting a new summary was mistakenly being interpreted as
an instance that should create a new TopicHistoryIndex entry,
but at any point a summary could be a submitted there must
already exist history.

Patch adjusts the callback that determines if a new index should
be created(rather than updated) to only create the index for
post revisions.

Bug: 67502
Change-Id: Ia42b695faacc3b2fab44ee7bf2bb90fb7713489c
Merge "Summarizing a topic makes previous history invisible"