Configure git to show commit body if one exists

This is especially useful for `git show`, but it also applies to `git log` and
`git whatchanged`.

This change uses the `%n`, `%b`, and `-` options, which are described in the Git
docs [1] as follows:

> %n: newline

> %b: body

> If you add a - (minus sign) after % of a placeholder, line-feeds that
> immediately precede the expansion are deleted if and only if the placeholder
> expands to an empty string.

[1]: https://www.kernel.org/pub/software/scm/git/docs/git-show.html