tutorial: revise index introduction

The embarassing history of this tutorial is that I started it without
really understanding the index well, so I avoided mentioning it.

And we all got the idea that "index" was a word to avoid using around
newbies, but it was reluctantly mentioned that *something* had to be
said.  The result is a little awkward: the discussion of the index never
actually uses that word, and isn't well-integrated into the surrounding
material.

Let's just go ahead and use the word "index" from the very start, and
try to demonstrate its use with a minimum of lecturing.

Also, remove discussion of using git-commit with explicit filenames.
We're already a bit slow here to get people to their first commit, and
I'm not convinced this is really so important.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>

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