While debugging another issue, I discovered that if a diff contains a

"new file" but has a parent diff that previously deleted that file, it

would show in two-column mode with the left-hand side totally empty.
This traced back to an old fix done way back in 2012, back when we only

had a source_revision field which was overloaded between both the diff

and parent diff. That was fixed in 3.0.19, but the is_new_file

calculation still completely turned it off if we had a parent diff

present at all.
I've changed this so that if we have a modern parent_source_revision,

we allow ourselves to trust the newfile flag.

Verified that this situation now correctly appeared as a new file in

  the diff viewer.
Ran unit tests.