incorrect TOTAL when diffing same binary

Since https://github.com/google/bloaty/pull/313/files (specifically https://github.com/google/bloaty/commit/7c5547aeac42917c0d58e19b77332cd07f122a02) 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.