Make 'Splitting a Commit' paragraph 2 more clear.

The first sentence in paragraph 2 of 'Splitting a Commit' did not
make much sense in the context of the rest of the paragraph. This
commit removes it and integrates the useful bits into a later point
where they flow more sensibly.
Merge pull request #364 from wcarss/ch6_remove_cruft_sentence

Remove cruft sentence from Chapter 6.