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.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).