[PATCH 3/5] lib-compression: Add o_stream_deflate_reset_dict()

[PATCH 3/5] lib-compression: Add o_stream_deflate_reset_dict()

New public function that locates the deflate ostream in the parent chain
(transparently handles rawlog wrappers) and schedules a Z_FULL_FLUSH on
the next uncork/flush.  Z_FULL_FLUSH emits all buffered data and then
resets the deflate dictionary, so subsequent compressed output cannot
reference data from before the call.

Adds a pending_dict_reset flag to struct zlib_ostream.  The flag persists
through partial flush retries and is cleared only once the full flush
loop completes.

Gbp-Pq: Name 0003-lib-compression-Add-o_stream_deflate_reset_dict.patch