subrepo: fix test breakage introduced in 2b34d004e644

The test didn't break when I originally committed it (git version
1.7.12.4 (Apple Git-37)), and still doesn't on that machine, but the
output changes elsewhere with "Warning: you are leaving N commits
behind..."

This change sets a ref on the commit we're updating the subrepo _away_
from to quash the warning.

tests/test-subrepo-git.t

@@ -253,6 +253,7 b' pulling new git branch should not create'
 (issue3870)
   $ cd ../td/s
   $ git remote set-url origin $TESTTMP/tb/s
+  $ git branch --no-track oldtesting
   $ cd ..
   $ hg pull -q ../tb
   $ hg up