diff-tree: fix up comparison of "interesting" sub-trees

We used to trigger the "interesting subdirectory" check for any
matching name that started with the same character series, regardless
of whether it had the matching slash or not.

diff-tree.c

@@ -209,6 +209,11 @@ static int interesting(void *tree, unsigned long size, const char *base)
 		if (pathlen > matchlen)
 			continue;
 
+		if (matchlen > pathlen) {
+			if (match[pathlen] != '/')
+				continue;
+		}
+
 		if (strncmp(path, match, pathlen))
 			continue;