Comments made on an interdiff of a newly-added file rendered only the

right-hand side of the diff, instead of both sides.
The comment fragment template decided between the one-column and

two-column layouts using comment.filediff.is_new, which is simply a

check for a source revision of PRE_CREATION. For interdiffs, that's

the wrong thing to check. A file added in one revision and then modified

in the next has both an original and a modified side when shown as an

interdiff, and both must be rendered. The same is true when showing a

file relative to a base commit.
The diff viewer got this right, using the computed is_new_file state

from get_diff_files(), which factors in the interfilediff and the base

filediff. The comment fragment template was never updated to match, so

the two diverged.
I've split that out into a get_is_new_file() helper, which both

get_diff_files() and the comment fragment code call, so these now use

the same logic. The chunk generator was already producing correct

interdiff chunks with both sides populated. Only the template was

discarding the original side.
Two colspans in the comment fragment template were hard-coded to 3,

spanning four columns of a two-column table when rendering a genuinely

new file. These are now computed like the others.
Since review e-mails share this code, comment fragments in e-mail were

affected by all of this as well.

Manually reproduced the bug and verified the fix.
Ran unit tests.