releasetools: Fix the target files diffing.

This CL refactors the non-A/B OTA generation code into a function,
so it allows adding common post-generation code for both flows.

It moves the code for target files diffing to common path, and fixes the
diffing functionality (broken due to the use of unzip pattern).

Test: Run ota_from_target_files with `--log_diff` flag to generate
      non-A/B and A/B OTAs respectively.
Change-Id: I92e5549a58344fc56a151a709c06534913d265d5