commit: prompt the user to set a default username on commit if necessary
Currently TortoiseHg does not ever prompt the user to set its default username. This means that is quite easy to make a commit with an undesired username. This is particularly true on Windows, where the default username is the current account name followed by "@" and the full network computer name.
With this change TortoiseHg checks whether a default username is set, and if not it prompts the user to set one (letting the user open the settings dialog right from the prompt).
If there is an username available for given repository (e.g. username is specified in repository or username is specified in environment variable) and default username will not be used anyhow there is no reason to force user to define it at that moment.
With this code user is prevented from committing code whenever default username configuration is empty...