git-difftool: use git-mergetool--lib for "--tool-help"

git-difftool: use git-mergetool--lib for "--tool-help"

The "--tool-help" option to git-difftool currently displays incorrect
output since it uses the names of the files in
"$GIT_EXEC_PATH/mergetools/" rather than the list of command names in
git-mergetool--lib.

Fix this by simply delegating the "--tool-help" argument to the
show_tool_help function in git-mergetool--lib.

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>