Because, formerly, comment threads were instantiated by low-level diff
builder instances, they needed to be parameterized with information that
was only relevant to the thread groups (namely the project name and the
parent index). This resulted in tight coupling between gr-diff-builder
and both gr-diff-comment-thread-group and gr-rest-api-interface.
To decouple the builder from these two implementations, thread creation
is relocated to the gr-diff level and a create function is passed down
into the builder.
Change-Id: I8c39518088af97a49a3c4f24bd32ae073ce24b76
614bec662b