test-pushvars: invoke shell script hook via `sh` for Windows

Invoking *.sh on Windows leads to the "what program should open this?" prompt,
which stalls the test and led to the recent series of exceptions on the Windows
test machine as the runner times out.

tests/test-pushvars.t

@@ -8,10 +8,9 b' Setup'
   > env | egrep "^HG_USERVAR_(DEBUG|BYPASS_REVIEW)" | sort
   > exit 0
   > EOF
-  $ chmod +x $TESTTMP/pretxnchangegroup.sh
   $ cat >> $HGRCPATH << EOF
   > [hooks]
-  > pretxnchangegroup = $TESTTMP/pretxnchangegroup.sh
+  > pretxnchangegroup = sh $TESTTMP/pretxnchangegroup.sh
   > [experimental]
   > bundle2-exp = true
   > EOF