That is, jj will use ui.default_description as a starting point when
user is about to describe an empty change.
I think it might be confusing to do this with -m / --stdin (violates
WYSIWYG), so I'm only doing this when jj invokes an editor.
Also, this could evolve into a proper template in the future instead of
just plain text, to allow inheriting from parent change(s), for example.
Partially addresses #1354.
