changegroup: call 'prechangegroup' hook before setting up write delay

The 'prechangegroup' interfere with 'delayupdate' logic because it trigger the
one time call of 'changelog._writepending' (see issure4934). There is no reason
not to call that hook before setting up 'delayupdate' so we move the call a bit
earlier to avoid interference.

mercurial/changegroup.py

@@ -329,13 +329,13 class cg1unpacker(object):
             # the top level value (if they exist) in this function.
             srctype = tr.hookargs.setdefault('source', srctype)
             url = tr.hookargs.setdefault('url', url)
+            repo.hook('prechangegroup', throw=True, **tr.hookargs)
 
             # write changelog data to temp files so concurrent readers
             # will not see an inconsistent view
             cl = repo.changelog
             cl.delayupdate(tr)
             oldheads = cl.heads()
-            repo.hook('prechangegroup', throw=True, **tr.hookargs)
 
             trp = weakref.proxy(tr)
             # pull off the changeset group