Move Client compression spec from constructor to new_packwriter().

This makes remote usage match local usage, where the compression is
per-PackWriter.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
split-cmd.py: respect --compress for remote repos too.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
save-cmd.py, split-cmd.py: end the commit message with a newline.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>