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

@@ -1,71 +1,70 b''
 Setup
 
   $ PYTHONPATH=$TESTDIR/..:$PYTHONPATH
   $ export PYTHONPATH
 
   $ cat > $TESTTMP/pretxnchangegroup.sh << EOF
   > #!/bin/sh
   > 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
 
   $ hg init repo
   $ hg clone -q repo child
   $ cd child
 
 Test pushing vars to repo with pushvars.server not set
 
   $ echo b > a
   $ hg commit -Aqm a
   $ hg push --pushvars "DEBUG=1" --pushvars "BYPASS_REVIEW=true"
   pushing to $TESTTMP/repo (glob)
   searching for changes
   adding changesets
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files
 
 Setting pushvars.sever = true and then pushing.
 
   $ echo [push] >> $HGRCPATH
   $ echo "pushvars.server = true" >> $HGRCPATH
   $ echo b >> a
   $ hg commit -Aqm a
   $ hg push --pushvars "DEBUG=1" --pushvars "BYPASS_REVIEW=true"
   pushing to $TESTTMP/repo (glob)
   searching for changes
   adding changesets
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files
   HG_USERVAR_BYPASS_REVIEW=true
   HG_USERVAR_DEBUG=1
 
 Test pushing var with empty right-hand side
 
   $ echo b >> a
   $ hg commit -Aqm a
   $ hg push --pushvars "DEBUG="
   pushing to $TESTTMP/repo (glob)
   searching for changes
   adding changesets
   adding manifests
   adding file changes
   added 1 changesets with 1 changes to 1 files
   HG_USERVAR_DEBUG=
 
 Test pushing bad vars
 
   $ echo b >> a
   $ hg commit -Aqm b
   $ hg push --pushvars "DEBUG"
   pushing to $TESTTMP/repo (glob)
   abort: unable to parse variable 'DEBUG', should follow 'KEY=VALUE' or 'KEY=' format
   [255]