revlog: fix caching of buffer objects

chunkiter: handle large reads more efficiently - for large reads, don't attempt to read more than necessary - if we've gathered the exact number of bytes needed, avoid a string copy

revlog: avoid large yields in group() Split large yields so that the downstream consumer (chunkbuffer) will avoid some pain when reading ahead.

chunkiter: simplify iter logic

chunkbuffer: removed unused method and arg

revlog: reduce memory usage in addgroup - use a buffer to extract the delta from a chunk - avoid concatenating to a compressed delta - use a buffer to directly extra full text from a trivial delta - delete chunk and delta objects after use

mpatch: allow buffer objects for input