Why is a good commit message important? Good communication about what was changed and why reduces the work involved in coming up to speed on a piece of code.
We recommend reading Chris Beams' great page on how to write a commit message. Here's a summary if you're in a hurry; each one is a link back to Chris' full description:
It's worth taking the time to keep it from being truncated.
This makes it clear that it's a subject line.
Save the space.
Git convention. This summary is written in the imperative.
If the subject and body are contiguous, tools such as log won't be able to distinguish them.
This allows a little space for indenting.
The code itself will explain how. If the code is too complex for that, it should have good commenting that does the job of explaining how.