Fix handling of revisions with deleted files for SVN.

The implementation of post-review did the wrong thing when the SVN revision had
a deleted file. Posting the diff manually worked fine, so the bug was entirely
in the post-review side.

This patch has been kicking around in our bug tracker for more than a year, in
formats which made it very hard to review. Multiple posters there vouched that
it worked.

Fixes bug 1113.

This patch has been kicking around in our bug tracker for more than a year, in
formats which made it very hard to review. Multiple posters there vouched that
it worked.