code clarity: one-letter var carried for too long
code clarity: one-letter var carried for too long

In split-cmd.py, the "w" variable is first seen on line 55 and is kept
around until line 96. Variables that are sparsely used in a medium
distance in the code should have a name that carries more sense when
read on its own.

Change "w" into "pack_writer" to better identify the purpose of the
variable.

Signed-off-by: Gabriel Filion <8df7619ec42e51274a2f3ff6cd1afa366f63857b@gmail.com>

mhoeher/bup,tjanez/bup,pombredanne/bup,tjanez/bup,gevaerts/bup,ToxicFrog/bup,jbaber/bup,tjanez/bup,apenwarr/bup,apenwarr/bup,pombredanne/bup,jbaber/bup,gevaerts/bup,ToxicFrog/bup,apenwarr/bup,ToxicFrog/bup,ToxicFrog/bup,gevaerts/bup,tjanez/bup,ToxicFrog/bup,gevaerts/bup,mhoeher/bup,tjanez/bup,mhoeher/bup,apenwarr/bup,jbaber/bup,pombredanne/bup,tjanez/bup,jbaber/bup,jbaber/bup,mhoeher/bup,mhoeher/bup,gevaerts/bup,mhoeher/bup,pombredanne/bup,jbaber/bup,pombredanne/bup,pombredanne/bup