[sdk-platform-java] Parse the release note from a BUILD-only commit in googleapis

[JoeWang1127]

Parse the release note from a BUILD-only commit in googleapis, e.g., googleapis/googleapis@de97a5f.

Modify https://github.com/googleapis/sdk-platform-java/blob/4d4c798feb6e9eee2c88fdbda1282d8c109b8eb5/hermetic_build/release_note_generation/generate_pr_description.py#L154

[blakeli0]

Adding a little more background info, we initially decided to not include BUILD file changes in the commit history generation because it could include changes from other languages. However, Java only changes to BUILD files are also excluded. We need to have a way to exclude changes from other languages in BUILD file, and only include changes from Java.