Update git-diff documentation

Porcelain documentation should talk in terms of end-user workflow, not
in terms of implementation details.  Do not suggest update-index, but
git-add instead.  Explain differences among 0-, 1- and 2-tree cases
not as differences of number of trees given to the command, but say
why user would want to give these number of trees to the command in
what situation.

Signed-off-by: Junio C Hamano <junkio@cox.net>

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