The purpose of a commit message is to summarize a change. But the purpose of summarizing a change is to help you and your team understand what is going on in your project. The information you put into a message, therefore, should be valuable and useful to the people who will read it.
As fun as it is to use the commit message space for cursing—at a bug, or Git, or your own clumsiness—avoid editorializing. Avoid the temptation to write a commit message like “Aaaaahhh stupid bugs.” Instead, take a deep breath, grab a coffee or some herbal tea or do whatever you need to do to clear your head. Then write a message that describes what changed in the commit, as clearly and succinctly as you can.