Write git pack files instead of loose object files.

Write git pack files instead of loose object files.

This causes much, much less disk grinding than creating zillions of files,
plus it's even more disk space efficient.

We could theoretically make it go even faster by generating the .idx file
ourselves, but for now, we just call "git index-pack" to do it.  That
helpfully also confirms that the data was written in a git-compatible way.