git-pickaxe: rename detection optimization

The idea is that we are interested in renaming into only one path, so
we do not care about renames that happen elsewhere.

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

builtin-pickaxe.c

@@ -289,6 +289,7 @@ static struct origin *find_rename(struct scoreboard *sb,
 	diff_opts.recursive = 1;
 	diff_opts.detect_rename = DIFF_DETECT_RENAME;
 	diff_opts.output_format = DIFF_FORMAT_NO_OUTPUT;
+	diff_opts.single_follow = origin->path;
 	paths[0] = NULL;
 	diff_tree_setup_paths(paths, &diff_opts);
 	if (diff_setup_done(&diff_opts) < 0)

diff.h

@@ -46,6 +46,7 @@ struct diff_options {
 	const char *filter;
 	const char *orderfile;
 	const char *pickaxe;
+	const char *single_follow;
 	unsigned recursive:1,
 		 tree_in_recursive:1,
 		 binary:1,

diffcore-rename.c

@@ -256,11 +256,15 @@ void diffcore_rename(struct diff_options *options)
 
 	for (i = 0; i < q->nr; i++) {
 		struct diff_filepair *p = q->queue[i];
-		if (!DIFF_FILE_VALID(p->one))
+		if (!DIFF_FILE_VALID(p->one)) {
 			if (!DIFF_FILE_VALID(p->two))
 				continue; /* unmerged */
+			else if (options->single_follow &&
+				 strcmp(options->single_follow, p->two->path))
+				continue; /* not interested */
 			else
 				locate_rename_dst(p->two, 1);
+		}
 		else if (!DIFF_FILE_VALID(p->two)) {
 			/* If the source is a broken "delete", and
 			 * they did not really want to get broken,