git.packwriter: explicitly manage lifetimes

git.packwriter: explicitly manage lifetimes

Explicitly close all packwriters, instead of relying on __del__.  Use
"with ..." context management in most cases..  This is in preparation
for removing __del__.

Rework _end to handle *all* cleanup and to ensure that we always
release the relevant resources, whether closing normally, or aborting.
This means that we only have two places to consider with respect to
resource management: __init__ and _end.

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