Compute exact sizes via t/data-size in test-gc.sh

Add a t/data-size helper that computes the total data size of all named
paths and use it instead of du in test-gc.sh to determine the
before/after repository sizes.

This should fix spurious test failures on filesystems with a large block
size.  (Observed on ppc64el and powerpc tmpfs filesystems.)

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