If we just wanted to extract the bug IDs from the commit messages and didn't care about the commit IDs, we could have just used grep after the git log command, still limiting the log to the last tag:
$ git log v3.1.0.201310021548-r..HEAD | grep "Bug: "
If we just want the commit IDs and their subjects, but not the actual bug IDs, we can use the --oneline feature of git log combined with the --grep option:
$ git log --grep "Bug: " --oneline v3.1.0.201310021548-r..HEAD