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.

mercurial/changegroup.py

@@ -406,6 +406,10 b' class cg1unpacker(object):'
                              % (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))

tests/test-hook.t

@@ -686,6 +686,7 b' new commits must be visible in pretxncha'
   $ 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 b' new commits must be visible in pretxncha'
   $ 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