viewvcs: support streaming large blobs

Forking off git-cat-file here for streaming large blobs is
reasonably efficient, at least no worse than using
git-http-backend for serving clones.  So let our limiter
framework deal with it.

git itself isn't great for large files, and AFAIK there's no
stable/widely-available mechanisms for reading smaller chunks
of giant blobs in git itself.

Tested with some giant GPU headers in the Linux kernel.

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