I also changed the instructions to use `cargo install --git` pointing
straight to GitHub, so we don't have the naming conflict with the jj
repo created in the tutorial.
The command's help text says "Abandon a revision", which I think is a
good indication that the command's name should be `abandon`. This
patch renames the command and other user-facing occurrences of the
word. The remaining occurrences should be removed when I remove
support for evolution.
