Resurrect git-revert.sh example and add comment to builtin-revert.c

I had to scratch my head for quite some time figuring out why we
cannot optimize out write_tree() we do when --no-commit option
is given, whose purpose seem to be only to check if the index is
unmerged, with a simple loop over the active_cache[].

So add a comment to describe why the write_tree() is there, and
resurrect the last scripted version as a reference material in
contrib/example directory with others.

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

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