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.