Which version of the book is affected?

Problem is present in source files. So naturally all that follows from the source files is also affected.

Describe the bug:

Not all example commit messages in the source files are using the imperative mood.

Steps to reproduce:

1. Go to https://github.com/progit/progit2/blob/master/book/03-git-branching/sections/basic-branching-and-merging.asc
   and search for: "fixed the broken email address".
2. Commit message is not written in the imperative mood (the way git talks to you on the command line).

Expected behavior:

I think it would set a good standard if the readers get to see "proper" commit messages throughout the book. This way the readers will get familiar with the correct way to write commit messages.

Screenshots:

Additional context:

Reference to our style guide for commits:
From https://github.com/progit/progit2/blob/master/book/05-distributed-git/sections/contributing.asc
Write your commit message in the imperative: "Fix bug" and not "Fixed bug" or "Fixes bug."

I'm willing to fix the issue, by making new pull-requests, if @ben wants to change the example commit messages too.

Related issue #972.