hooks: fix hooks not firing if prechangegroup was set (issue4934) stable

hooks: fix hooks not firing if prechangegroup was set (issue4934)

We need to call delayupdate again after writing to the changelog.
Otherwise the prechangegroup hook consumes the delayupdate subscription and
future hooks don't see the pending changes (see issue 4934 for more details).

Adds a test that triggers the prechangegroup hook before the pretxnchangegroup
hook and verifies that the output of pretxnchangegroup doesn't change.

--- a/mercurial/changegroup.py	Tue Nov 03 16:58:13 2015 -0800
+++ b/mercurial/changegroup.py	Tue Nov 03 17:13:27 2015 -0800
@@ -406,6 +406,10 @@
                              % (changesets, revisions, files, htext))
             repo.invalidatevolatilesets()
 
+            # Call delayupdate again to ensure the transaction writepending
+            # subscriptions are still in place.
+            cl.delayupdate(tr)
+
             if changesets > 0:
                 if 'node' not in tr.hookargs:
                     tr.hookargs['node'] = hex(cl.node(clstart))

--- a/tests/test-hook.t	Tue Nov 03 16:58:13 2015 -0800
+++ b/tests/test-hook.t	Tue Nov 03 17:13:27 2015 -0800
@@ -686,6 +686,7 @@
   $ cd ..
   $ hg init to
   $ echo '[hooks]' >> to/.hg/hgrc
+  $ echo 'prechangegroup = hg --traceback tip' >> to/.hg/hgrc
   $ echo 'pretxnchangegroup = hg --traceback tip' >> to/.hg/hgrc
   $ echo a >> to/a
   $ hg --cwd to ci -Ama
@@ -698,6 +699,12 @@
   $ hg --cwd from push
   pushing to $TESTTMP/to (glob)
   searching for changes
+  changeset:   0:cb9a9f314b8b
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     a
+  
   adding changesets
   adding manifests
   adding file changes