For a new user, it is not clear how to view the full commit
message/description of a change with `jj log`.
This fix this, add a new template alias
`builtin_log_compact_full_description` to display the commit like
`builtin_log_compact` does but with a full description.
The user can set it to true on the config like this:
```
templates.log = builtin_log_compact_full_description
```
Fixes: #3688