Becky Siegel 562a30493e Allow multiple threads per line, with different ranges
- Add concept of diff comment thread groups, which are all of the
  threads at a particular line number.

- The thread group is responsible for breaking up comments into threads
  based on the range of the comment.

- Thread groups are ordered by the updated time of the first comment in
  the group.

- Thread groups are given a key, based on comment range, which is used
  to determine what thread group a new comment should go in (or if it
  needs a new one).

Feature: Issue 5292
Change-Id: If544e8bb879262de3ce5397e86124837b66ada04
2017-02-01 16:10:08 -08:00
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