match: bound the hash_search() chain walk (issue #217)

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hash_search() walks the entire hash-table chain for the current rolling
checksum at every byte offset of the source file. Disk and VM images
contain large runs of identical blocks, so a single weak checksum
(get_checksum1) can collide thousands of times and pile every one of
those blocks onto one chain. When the sender then rolls across a region
whose weak checksum keeps landing on that chain without ever producing a
strong-checksum match, it re-walks the whole chain for every byte, giving
O(file_size * chain_length) behaviour. The result is rsync sitting at
100% CPU for hours with no apparent progress -- the long-standing "rsync
hangs on large files" reports.

Cap the number of same-weak-checksum candidates examined per offset at
MAX_CHAIN_LEN. Once the cap is hit we treat the offset as a non-match and
roll forward a byte; any block skipped this way is simply sent as literal
data, so the transferred result is always correct -- only the transfer
size is marginally affected. This is purely a sender-side search limit:
it changes no checksum, emitted byte, or protocol field, so a capped
sender interoperates with an unmodified receiver and vice versa.

On a synthetic 40000-block basis sharing one weak checksum, syncing a
60KB source dropped from ~18.4s to ~0.7s; the unbounded cost grows with
the square of the file size.

testsuite/hashsearch-chain_test.py reproduces the pathology with a tiny
basis of weak-checksum-colliding decoy blocks and asserts, via the
existing false_alarms counter (--debug=deltasum1), that the per-hash-hit
chain walk stays bounded. The assertion is exact and machine-independent
rather than timing-based.

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