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
#header#
# HG changeset patch
# User #author#
# Date #date|hgdate#
# Node ID #node#
#parent%changesetparent#
#desc#
#diff#