Don't hard-code /tmp in test-ro-message

This makes the test succeed when $TMPDIR is set.

tests/test-ro-message

@@ -1,17 +1,17
 #!/bin/sh
 HG=hg
 "$HG" init
 mkdir b
 echo 'Bouncy' >b/bouncy
 echo 'tricycle' >b/vehicle
 "$HG" add b/bouncy
 "$HG" add b/vehicle
 "$HG" commit -m 'Adding bouncy'
 echo 'bouncy' >>b/bouncy
 "$HG" commit -m 'Making it bouncier'
 "$HG" update -C 0
 echo 'stationary' >>b/vehicle
 "$HG" commit -m 'Clarifying the vehicle.'
 "$HG" update -C 1
 chmod a-w b/vehicle
-"$HG" update -m 2 2>&1 | sed 's|^\(.*[ 	]\)/tmp/[^/]*/\(.*\)$|\1\2|g'
+"$HG" update -m 2 2>&1 | sed 's|^\(.*[ 	]\).*/\([^/]*/[^/]*/[^/]*\)$|\1\2|g'