Since git version 1.7.6, git config has gained a log.abbrevCommit option which can be set to true. Thus the answer is upgrade to at least 1.7.6 (current as of this writing is 1.7.11.4) and use:
git config --global log.abbrevCommit true
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…