Command-line tips from blog post

[ben]

Mentioned in #1, this blog post has some good tips that we should add:

• git log -S'pattern'
• git log ..BRANCH
• git branch --contains
• git shortlog -sn

[schacon]

the first two are now in the book, I don't really think the third needs to be and if you want to add a short section on shortlog you can, but I rarely use it.