git-mergetool: don't hardcode 'mergetool' in show_tool_help

When using show_tool_help from git-difftool we will want it to print
"git difftool" not "git mergetool" so use "git ${TOOL_MODE}tool".

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>