test-lib: Work around missing sum on Windows

t1002-read-tree-m-u-2way.sh uses 'sum', but it does not rely on the exact
form of the sum, only that it is a hash digest. Therefore, we can sneak
in 'md5sum' under the name 'sum'.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>

t/test-lib.sh

@@ -646,5 +646,8 @@ case $(uname -s) in
 	find () {
 		/usr/bin/find "$@"
 	}
+	sum () {
+		md5sum "$@"
+	}
 	;;
 esac