The UI does not allow creating multiple comments at the same diff
location, but instead encourages contributing to the comment threads
already at a location. The uniqueness of thread locations was leveraged
to simplify the collection of comments into threads by mapping their
locations to keys.
However, the REST API does permit multiple threads starting from the
same diff location, so this expectation in the key-based UI threading
logic can be wrong. In such scenarios, an unresolved thread may be
mistakenly inserted into a resolved thread, resulting in a comment that
is not resolvable through the UI.
With this change, the threading logic in gr-diff-comment-thread-group is
updated to associate comments into threads via the `in_reply_to` graph
rather than location map in the same way that the
`unresolved_comment_count` detail property is determined server-side.
Bug: Issue 6779
Change-Id: Ibb29d4ba5c5de9e92dfbfba8ac7ac8037c332028
f78a45c7eb