test-tag: make hook runnable on Windows

Simply setting the hook to *.sh causes Windows to open a dialog asking how to
open *.sh files.

tests/test-tag.t

@@ -2,7 +2,7 b''
   > [experimental]
   > hook-track-tags=1
   > [hooks]
-  > txnclose.track-tag=${TESTTMP}/taghook.sh
+  > txnclose.track-tag=sh ${TESTTMP}/taghook.sh
   > EOF
 
   $ cat << EOF > taghook.sh
@@ -14,7 +14,6 b''
   >     sed 's/^/hook: /' .hg/changes/tags.changes
   > fi
   > EOF
-  $ chmod +x taghook.sh
   $ hg init test
   $ cd test