When the diff processor collapses shared groups based on the user
context preference, it would sometimes collapse a single line, resulting
in a common group labeled "Show 1 common line" which takes up exactly
as much space in the diff view as the line itself. Moreover, the GWT UI
does not collapse single line common sections in this way (but does when
there are two or more lines).
Sets the threshold to collapse at least two lines and encodes this in a
test.
Bug: Issue 4721
Change-Id: I8a3a8556fd66a313eacc361379612c700d7ae070
9d8649bfa1