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

adding bar
adding foo
% state dump
a   0         -1 baz
n 644          0 foo
r   0          0 bar
% status
A baz
R bar
C foo
% state dump
n 666         -1 bar
n 666         -1 foo
% status
! bar
? baz
C foo