Fix tree-walking compare_entry() in the presense of --prefix

When we make the "root" tree-walk info entry have a pathname in it, we
need to have a ->prev pointer so that compare_entry will actually notice
and traverse into the root.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree-walk.c

@@ -107,6 +107,7 @@ int tree_entry(struct tree_desc *desc, struct name_entry *entry)
 void setup_traverse_info(struct traverse_info *info, const char *base)
 {
 	int pathlen = strlen(base);
+	static struct traverse_info dummy;
 
 	memset(info, 0, sizeof(*info));
 	if (pathlen && base[pathlen-1] == '/')
@@ -114,6 +115,8 @@ void setup_traverse_info(struct traverse_info *info, const char *base)
 	info->pathlen = pathlen ? pathlen + 1 : 0;
 	info->name.path = base;
 	info->name.sha1 = (void *)(base + pathlen + 1);
+	if (pathlen)
+		info->prev = &dummy;
 }
 
 char *make_traverse_path(char *path, const struct traverse_info *info, const struct name_entry *n)