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 `getCommentThreads` method 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.
Although this corrects the issue in the `getCommentThreads` method, as
of this change, the diffs do not yet use it to thread comments.
Migrating diffs over to use it will be done in follow-up.
Bug: Issue 6779
Change-Id: I9427ccc716acdf7ef4c63eba48d2287875fa5534
fdf0db7a2b