debugshell: allow commands to be specified as a CLI argument
Add a `--command` option to `hg debugshell` that allows the user to pass in
Python code to evaluate directly from the command line. This was inspired by
the `--command` option present in Facebook's Sapling fork of Mercurial,
which in turn was inspired by the `-c` option of the Python interpreter
itself. It is particularly useful for writing tests, especially for getting
visibility into things that otherwise aren't exposed via debug commands.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true