diff-tree --cc: squelch header generation on empty patch.

diff-tree --cc: squelch header generation on empty patch.

Earlier round showed the commit log header and "diff --combined"
header even for paths that had no interesting hunk under --cc
flag.  Move the header display logic around to squelch them.
With this, a merge that does not have any interesting merges
will not be shown with --cc option, unless -m is used at the
same time.

Signed-off-by: Junio C Hamano <junkio@cox.net>