changegroup: fix the scope of a try finally

The try finally is here to ensure we release the just-created transaction.
Therefore we should not do half a dozen operations before actually entry the try
scope.

mercurial/changegroup.py

@@ -322,6 +322,7 b' class cg1unpacker(object):'
         changesets = files = revisions = 0
 
         tr = repo.transaction("\n".join([srctype, util.hidepassword(url)]))
+        try:
         # The transaction could have been created before and already
         # carries source information. In this case we use the top
         # level data. We overwrite the argument because we need to use
@@ -329,12 +330,11 b' class cg1unpacker(object):'
         srctype = tr.hookargs.setdefault('source', srctype)
         url = tr.hookargs.setdefault('url', url)
 
-        # write changelog data to temp files so concurrent readers will not see
-        # inconsistent view
+            # write changelog data to temp files so concurrent readers
+            # will not see an inconsistent view
         cl = repo.changelog
         cl.delayupdate(tr)
         oldheads = cl.heads()
-        try:
             repo.hook('prechangegroup', throw=True, **tr.hookargs)
 
             trp = weakref.proxy(tr)