difftool: Allow specifying unconfigured commands with --extcmd

difftool: Allow specifying unconfigured commands with --extcmd

git-difftool requires difftool.<tool>.cmd configuration even when
tools use the standard "$diffcmd $from $to" form.  This teaches
git-difftool to run these tools in lieu of configuration by
allowing the command to be specified on the command line.

Reference: http://article.gmane.org/gmane.comp.version-control.git/133377
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>