Previously diff fragments of a rebased interdiff would sometimes not be

renderable. This was due to there being blank lines in chunks because of

the upstream changes being filtered out. This broke some assumptions

about the existence of line numbers in equals chunks.
We are now more careful about our assumptions and check for the presence

of the line numbers themselves instead of checking the chunk's change

type. The diff fragments now render correctly. A test has been added to

ensure that this does not regress.
We also are more careful when determining headers that can be

expanded to for a fragment because when chunks get merged together by

the opcode generator, the headers may end up in the wrong chunk. A test

has been added to ensure that the correct headers are returned when this

occurs.
Diff header expansion now correctly compares the line numbers of each

header. Previously, the virtual element was being compared (instead of

line), which wasn't a key in either dict and so the comparsion was

being ignored silently. This results in left and right expansion headers

with different lines with the same code correctly being seperated.
I also noticed that the terms virtual line number and unified number

were being used to refer to the same thing, so all references to unified

line numbers have been changed to refer to virtual line numbers for

consistency's sake.

Ran unit tests.
Uploaded two diffs to my development server. The second diff was rebased

off a later change and then modified. I created an issue on a section

just after a half blank chunk in the interdiff. The fragment was not

rendered without the patch applied and rendered correctly with the patch

applied. The expansion links also worked correctly with the patch

applied.
The provided tests fail without this patch applied.