incorrect TOTAL when diffing same binary

[bence42]

Since https://github.com/google/bloaty/pull/313/files (specifically 7c5547a) the diff will incorrectly contain the new total size.

E.g

$ ./build/bloaty ./build/bloaty -- ./build/bloaty
    FILE SIZE        VM SIZE
 --------------  --------------
  +100% +7.30Mi  +100% +7.55Mi    TOTAL

Previously the report was correct:

$ bloaty /usr/bin/bloaty -- /usr/bin/bloaty
    FILE SIZE        VM SIZE
 --------------  --------------
  [ = ]       0  [ = ]       0    TOTAL

This make diff almost unusable because the actual TOTAL change is not calculated.