Feature Request: harness option `--local-changes=error` or similar to check if there are local changes

[hagertnl]

Proposing a flag like:

runtests.py --local-changes=[ignore,warn,exit]

to give the user some customization over how to behave when there's local changes in a Git repo. For example, you can either ignore these changes, print a message to warn about the changes (which would be default behavior), or abort the application runs if there are non-committed local changes. I think this would be useful for ensuring consistency of tests being run.