tutorial: clarify "pull" is "fetch + merge"

The document says that a fetch with a configured remote stores what are
fetched in the remote tracking branches "Unlike the longhand form", but
there is no longhand form "fetch" demonstrated earlier.

This adds a missing demonstration of the longhand form, and a new
paragraph to explain why some people might want to fetch before pull.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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