This both helps find the current checkout and head operation and
hopefully helps teach the user that "@" is the symbol for the working
copy. I removed the current "<--" indication from the graph (and
non-graph) log template. Hopefully the "@" is clear enough on its own,
but we may want to add back some further indication later. We'll see.
I considered even changing the message to "Checking out: <commit>" as
that's technically more correct (the message is printed when the
view's checkout is updated, i.e. before the working copy is
updated). However, I worried that users would find it confusing that
e.g. `jj close` would result in a "Checking out: " message, even
though that's what actually happens.
I remember adding that message a long time ago so the user has a trace
of working copy commit ids in the terminal output. They should be able
to get the same information from the operation log combined with
e.g. `jj st --at-op`.
The project is starting to work well enough that I think it's time
that the documentation targets users instead of VCS hackers. This
patch rewrites much of the README to describe how the features help
the user instead of describing how they work. It also adds a tutorial.
