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 bf73fc21

 (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>