tests: on windows, run command explicitly in sh for working command substitution

On windows, os.system runs the command in cmd.exe, which does not know command
substitution. To let this work, run the command explicitly in sh.

This fixes the failure of this test on windows.

tests/test-commandserver.py

@@ -267,7 +267,10 b' def obsolete(server):'
 
     runcommand(server, ['up', 'null'])
     runcommand(server, ['phase', '-df', 'tip'])
-    os.system('hg debugobsolete `hg log -r tip --template {node}`')
+    cmd = 'hg debugobsolete `hg log -r tip --template {node}`'
+    if os.name == 'nt':
+        cmd = 'sh -c "%s"' % cmd # run in sh, not cmd.exe
+    os.system(cmd)
     runcommand(server, ['log', '--hidden'])
     runcommand(server, ['log'])