mergetool: support --tool-help option like difftool does

This way we do not have to risk the list of tools going out of sync
between the implementation and the documentation.

In the same spirit as bf73fc2 (difftool: print list of valid tools
with '--tool-help', 2012-03-29), trim the list of merge backends in
the documentation.  We do not want to have a complete list of valid
tools; we only want a list to help people guess what kind of things
the tools do to be specified there, and refer them to --tool-help
for a complete list.

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

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