tests: use positive revision numbers in test-strip-cross.t

It took me a long time to realize that '-r -1' was pulling revision -1
because I didn't even notice the minus sign until I tried changing the
revision number. The order of arguments doesn't matter, so I changed
from decreasing order to increasing while at it.

Differential Revision: https://phab.mercurial-scm.org/D6790

tests/test-strip-cross.t

@@ -31,7 +31,7 b' 2 1 0 2 0 1 2'
   $ commit 'manifest-file'
   $ commit '012' 'manifest-file'
   $ cd ..
-  $ hg clone -q -U -r -1 -r -2 -r -3 -r -4 -r -6 orig crossed
+  $ hg clone -q -U -r 4 -r 6 -r 7 -r 8 -r 9 orig crossed
   $ cd crossed
   $ hg debugindex --manifest
      rev linkrev nodeid       p1           p2