memtest.py: a standalone program for testing memory usage in PackIndex.

memtest.py: a standalone program for testing memory usage in PackIndex.

The majority of the memory usage in bup split/save is now caused by
searching pack indexes for sha1 hashes.  The problem with this is that, in
the common case for a first full backup, *none* of the object hashes will be
found, so we'll *always* have to search *all* the packfiles.  With just 45
packfiles of 200k objects each, that makes about (18-8)*45 = 450 binary
search steps, or 100+ 4k pages that need to be loaded from disk, to check
*each* object hash.  memtest.py lets us see how fast RSS creeps up under
various conditions, and how different optimizations affect the result.