tests: load showstack in test-pull-http.t so network hangs are easier to find

This also gives us some minimal "it loads" coverage on showstack,
which I rather like. showstack doesn't work on Windows per mbarbison,
so it's disabled there.

I added this in service of debugging a hang introduced on Python 3 by
revision a88d68dc3ee8. I'm still not sure what the problem there is,
but this at least gives us a little bit of a chance to figure out
what's going on.

Differential Revision: https://phab.mercurial-scm.org/D3247

tests/test-pull-http.t

@@ -1,5 +1,16 b''
 #require killdaemons
 
+#if no-windows
+For debugging: this is a pretty simple test that is a good candidate
+for tracking down network-related bugs. Sometimes a command in this
+hangs, so having showstack pre-loaded is sometimes helpful. This also
+gives us a test that at least proves showstack can be loaded.
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > showstack = $TESTDIR/../contrib/showstack.py
+  > EOF
+#endif
+
   $ hg init test
   $ cd test
   $ echo a > a