scripts/bloat-o-meter: fix percent on <1% changes

Python divisions are integer divisions unless at least one parameter is
a float.  The current bloat-o-meter fails to print sub-percentage
changes:

  Total: Before=10515408, After=10604060, chg 0.000000%

Force float division by using one float and pretty the print to two
significant decimals:

  Total: Before=10515408, After=10604060, chg +0.84%

Link: http://lkml.kernel.org/r/1465980311-23814-1-git-send-email-riku.voipio@linaro.org
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>