Offer blame only where it works

The blame page answers 404 for a symlink, since git blames a regular
file alone, and it refuses a binary file and one past max-blob-size,
yet a tree listing offered blame for all of them, and so did a
symlink's own page. Each row already knows its size, and git calls a
file binary by its first 8000 bytes alone, which a stream reads
without inflating the rest. With blame on, a listing of 2,000
delta-packed files went from about 11 to 15 ms.

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