Show warning do not crash for missing version.cfg.

Also use RawConfigParser to avoid "%" interpolations.
Add fallback version.  Support commit metadata for git archive.

Switch to RawConfigParser to avoid issues with "%" interpolation.
Add commit metadata that expand for git tarball.
Improve resolution of version metadata.

Use gitarchive.cfg first, then git describe.
Do not export git-related files.
Avoid rewriting version.cfg with empty git data.
Run egg_info when git commit changes.
Keep GIT_DIR in the egg_info environment.