Extend parse_difftree_raw_line to save commit info from when
git-diff-tree is given only one <tree-ish>, for example when fed
from git-rev-list using --stdin option.
git-diff-tree outputs a line with the commit ID when applicable.
Signed-off-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <junkio@cox.net>