The jujutsu backend for rbtools gets the git diff and then processes it

in order to turn short blob hashes into full hashes. This was broken for

deleted files in two ways:

The regex that parsed index lines didn't catch deleted lines, because

  they didn't have any mode information.
Trying to turn '00000000' into a full hash would fail because that

  hash doesn't actually exist in the repo.

This change fixes those two issues and adds a test to verify it.
This also fixes a test for an updated message in jj.

Ran unit tests.
Posted a change that included a deleted file.