Add comment about this test failing under bzr 1.13 due to a bug in bzr.

tests/test-convert-bzr-merges

@@ -1,5 +1,9 b''
 #!/bin/sh
 
+# N.B. bzr 1.13 has a bug that breaks this test.  If you see this
+# test fail, check your bzr version.  Upgrading to bzr 1.13.1
+# should fix it.
+
 . "$TESTDIR/bzr-definitions"
 
 echo % test multiple merges at once