Conditional newline in pretty format

[Robert Dailey <rcdailey.lists@gmail.com>]

I have the following alias:

```
[alias]
    release-notes = log --no-merges --pretty=format:'* %s%n%n%w(100,2,2)%b'
```

With a branch `topic` checked out, I run it like so:

    $ git release-notes origin..

This gives me the "release notes" on my branch. The goal is to have it
formatted as a bullet-point list in markdown format so I can just copy
& paste this output into Github, Bitbucket, or other sites and have it
already set up for render correctly.

It works perfectly right now except for the case where `%b` is empty.
In that case, I just want one newline after `%s` instead of 2. Is
there a way to make my second `%n` conditional on `%b` having a value?