diff.c: do not use pathname comparison to tell renames

The final output from diff used to compare pathnames between
preimage and postimage to tell if the filepair is a rename/copy.
By explicitly marking the filepair created by diffcore_rename(),
the output routine, resolve_rename_copy(), does not have to do
so anymore.  This helps feeding a filepair that has different
pathnames in one and two elements to the diff machinery (most
notably, comparing two blobs).

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

diff.c

@@ -1786,13 +1786,9 @@ struct diff_filepair *diff_queue(struct diff_queue_struct *queue,
 				 struct diff_filespec *one,
 				 struct diff_filespec *two)
 {
-	struct diff_filepair *dp = xmalloc(sizeof(*dp));
+	struct diff_filepair *dp = xcalloc(1, sizeof(*dp));
 	dp->one = one;
 	dp->two = two;
-	dp->score = 0;
-	dp->status = 0;
-	dp->source_stays = 0;
-	dp->broken_pair = 0;
 	if (queue)
 		diff_q(queue, dp);
 	return dp;

diffcore-rename.c

@@ -205,6 +205,7 @@ static void record_rename_pair(int dst_index, int src_index, int score)
 	fill_filespec(two, dst->sha1, dst->mode);
 
 	dp = diff_queue(NULL, one, two);
+	dp->renamed_pair = 1;
 	if (!strcmp(src->path, dst->path))
 		dp->score = rename_src[src_index].score;
 	else

diffcore.h

@@ -53,11 +53,12 @@ struct diff_filepair {
 	char status; /* M C R N D U (see Documentation/diff-format.txt) */
 	unsigned source_stays : 1; /* all of R/C are copies */
 	unsigned broken_pair : 1;
+	unsigned renamed_pair : 1;
 };
 #define DIFF_PAIR_UNMERGED(p) \
 	(!DIFF_FILE_VALID((p)->one) && !DIFF_FILE_VALID((p)->two))
 
-#define DIFF_PAIR_RENAME(p) (strcmp((p)->one->path, (p)->two->path))
+#define DIFF_PAIR_RENAME(p) ((p)->renamed_pair)
 
 #define DIFF_PAIR_BROKEN(p) \
 	( (!DIFF_FILE_VALID((p)->one) != !DIFF_FILE_VALID((p)->two)) && \