Example git commit message

Add git commit message documentation (LP 9221421)

Describe how to write a good commit message by example.

Note that the first line should be a brief summary followed by the task
id.  This should be followed by a blank line, then a more detailed
explanation of the changes.

Furthermore, point out that the commit message should be describe the
changes in the code.  This helps those reviewing the code prior to
merging as well as future maintainers investigating why changes were
made.  When composing the commit message, the author should assume that
the commit message is providing the context the reader needs to
understand why the change was made.

Hard wrap the commit message.

Finally, note that two or more unrelated changes should not be contained
in the same commit.  This includes reformatting of existing code with
new logical changes; give the formatting changes their own commit.
Composing a good commit message can help identify commits that should be
split up.  Learn to use `git add -p`.