Linus Torvalds describes a good commit message. See https://github.com/torvalds/subsurface-for-dirk/blob/a48494d2fbed58c751e9b7e8fbff88582f9b2d02/README#L88-L115

commit-message-style-guide.txt

A good commit message looks like this:

	Header line: explaining the commit in one line

	Body of commit message is a few lines of text, explaining things
	in more detail, possibly giving some background about the issue
	being fixed, etc etc.

	The body of the commit message can be several paragraphs, and
	please do proper word-wrap and keep columns shorter than about
	74 characters or so. That way "git log" will show things
	nicely even when it's indented.

	Reported-by: whoever-reported-it
	Signed-off-by: Your Name <youremail@yourhost.com>

where that header line really should be meaningful, and really should be
just one line.  That header line is what is shown by tools like gitk and
shortlog, and should summarize the change in one readable line of text,
independently of the longer explanation.

Why signed-off commits:

If you want to contribute code, please either send signed-off patches or
a pull request with signed-off commits. If you don't sign off on them,
we will not accept them. This means adding a line that says
"Signed-off-by: Name " at the end of each commit, indicating that
you wrote the code and have the right to pass it on as an open source
patch.

See: http://developercertificate.org/

https://github.com/torvalds/subsurface-for-dirk/blob/a48494d2fbed58c751e9b7e8fbff88582f9b2d02/README#L79-L86

First version of the guide written by Linus: torvalds/subsurface-for-dirk@b659015