git-last-modified doesn't memset() full scratch bitmap

It was reported 1 the tests in t8020 fail on s390x. After some research, it seems it was related to s390x being big-endian. Well, actually, not really. Using big-endian simply uncovered the problem in test.

By not fully memset()ing the full buffer to zeroes might cause some bits keeping stuck. If they are stuck, paths might get attributed to the wrong commit.