In my previous change that updated the patching infrastructure to add

support for patching binary files, I changed the top-level Patcher class

so it would (when supported) call apply_patch_for_empty_files. This

caused a regression with Jujutsu, where we had no implementation of

that. I didn't catch this because it was hidden behind a conditional

which was false in the case of unit tests, since we weren't

instantiating the client with a capabilities object.
This change adds an implementation of that method for Jujutsu which

works similar to the SVN and Perforce implementations. Mercurial and Git

are otherwise immune because they use SCM-native patching which handles

empty files itself (hg import and git apply, respectively).
Some of the jj unit tests were incorrectly named, saying they included

empty files when they didn't. I've fixed those up, deleted a test which

did not do anything unique, and added two new tests similar to ones we

have in GitPatcherTests for ensuring that empty files work correctly.
To be complete, I've also added the default capabilities object to the

Git, Mercurial, and Perforce tests, just to ensure that the result of

supports_empty_files() will be correct when running the tests.

Ran unit tests.