This is a convenience optimization to improve the default user
experience, since `jj log` is a frequently run command. Accessing the
help information explicitly still follows normal CLI conventions, and
instructions are displayed appropriately if the user happens to make a
mistake. Discoverability should not be adversely harmed.
Note that this behavior mirrors what Sapling does [2], where `sl` will
display the smartlog by default.
[1] https://github.com/clap-rs/clap/issues/975
[2] https://sapling-scm.com/docs/overview/smartlog