combine-diff: finishing touches to git-diff-tree --cc

combine-diff: finishing touches to git-diff-tree --cc

This updates the output format to make administrative lines more
consistent with the traditional diffs.

The "index" line shows blob object names from each parents
(separated by commas), double dots and the object name of the
resulting blob.

The hunk header line begins with N+1 '@' characters for N-way
diff, the line number L of the first line in the hunk and line
count C from the parent in "-L,C" format for each parents and
then the line number of the first line in the hunk and line
count from the resulting file in "+L,C" format, and finally
N+1 '@' characters (earlier versions had the line numbers from
the resulting file at the beginning).

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