diff-tree: don't match non-directories as partial pathnames

This normally doesn't matter, but if you have a filename that is
sometimes a directory and sometimes a regular file (or symlink),
we don't want the regular file case to trigger a "partial match".

diff-tree.c

@@ -204,6 +204,8 @@ static int interesting(void *tree, unsigned long size, const char *base)
 		if (matchlen > pathlen) {
 			if (match[pathlen] != '/')
 				continue;
+			if (!S_ISDIR(mode))
+				continue;
 		}
 
 		if (strncmp(path, match, pathlen))