doc/README

util.chunkbuffer: avoid extra mutations when reading partial chunks

Previously, a read(N) where N was less than the length of the first
available chunk would mutate the deque instance twice and allocate a new
str from the slice of the existing chunk. Profiling drawed my attention
to these as a potential hot spot during changegroup reading.

This patch makes the code more complicated in order to avoid the
aforementioned 3 operations.

On a pre-generated mozilla-central gzip bundle, this series has the
following impact on `hg unbundle` performance on my MacBook Pro:

before: 358.21 real       317.69 user        38.49 sys
after:  301.57 real       262.69 user        37.11 sys
delta:  -56.64 real       -55.00 user        -1.38 sys

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