clean git workspaces

[mcandre]

As a developer, I want to setup a git clean task as a subtask of the usual project clean task. Because this helps save disk space when managing several git repository clones, and because when used safely, it can help to troubleshoot software problems.

Explore the options available to the git clean command.

https://git-scm.com/docs/git-clean

Add a git clean... command to makefiles, or whichever build system configuration file implements cleaning tasks, across my projects.

Ensure that as a precondition, git clean... should refuse to run, and should soft assert (forcibly exit zero), whenever the git clone has uncommitted changes.

https://stackoverflow.com/questions/27247123/how-do-i-clean-up-the-project-files-generated-by-cmake/27254365?noredirect=1#comment139017336_27254365