test-revlog-raw: fix "genbits" implementation
The "genbits" implementation is actually incorrect. This patch fixes it. A
good "genbits" implementation should pass the below assertion:
n = 3 # or other number
l = list(genbits(n))
assert 2**(n*2) == len(set((l[i]<<n)+l[i+1] for i in range(len(l)-1)))
An assertion is added to make sure "genbits" won't work unexpectedly.
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