* Misc size regression script improvements.
- Sets environment variable SIZE_REGRESSION_EXECUTING during execution
so hook scripts like `post-checkout` may skip processing.
- Forces checkout of the target branch, including removal of all
temporary object files in the process.
- Prints out a warning on exit stating that the git repository is in an
indeterminate state, and the user needs to swap back to whatever
intended branch they were working with originally.
* Remove `git clean`
