dispatch: add ability to specify a custom pdb module as a debugger This adds the ability to specify a config option, ui.debugger, to a custom pdb module, such as ipdb, and have mercurial use that as its debugger. As long as the value of ui.debugger is a loadable module with the set_trace and post_mortem functions, then dispatch will be able to use the custom module. Debugging _parseconfig is still available in the case of an error since it will be caught with a default the value of pdb.post_mortem.