diff-index --cached --raw: show tree entry on the LHS for unmerged entries.

This updates the way diffcore represents an unmerged pair
somewhat.  It used to be that entries with mode=0 on both sides
were used to represent an unmerged pair, but now it has an
explicit flag.  This is to allow diff-index --cached to report
the entry from the tree when the path is unmerged in the index.

This is used in updating "git reset <tree> -- <path>" to restore
absense of the path in the index from the tree.

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

diff.h

@@ -144,7 +144,9 @@ extern void diff_change(struct diff_options *,
 			const char *base, const char *path);
 
 extern void diff_unmerge(struct diff_options *,
-			 const char *path);
+			 const char *path,
+			 unsigned mode,
+			 const unsigned char *sha1);
 
 extern int diff_scoreopt_parse(const char *opt);