hgweb: catch util.Abort raised by addchangegroup

Right now, if a pretxnchangegroup hook fails, we send some HTML
error message to the client and the transaction is not rolled back
(issue499).

Catching util.Abort allows us to send a decent message to the client
and for some reason makes the rollback complete.

This patch is not perfect since it doesn't fix the reason why the
transaction wasn't rolled back (maybe some circular references?).
Also, the transaction is aborted only after we've sent the response
back to the client and the "transaction aborted" message ends up in
the logs of the web server.

mercurial/hgweb/hgweb_mod.py

@@ -1147,8 +1147,12 b' class hgweb(object):'
                 try:
                     url = 'remote:%s:%s' % (proto,
                                             req.env.get('REMOTE_HOST', ''))
-                    ret = self.repo.addchangegroup(util.chunkbuffer(gen),
-                                                   'serve', url)
+                    try:
+                        ret = self.repo.addchangegroup(util.chunkbuffer(gen),
+                                                       'serve', url)
+                    except util.Abort, inst:
+                        sys.stdout.write("abort: %s\n" % inst)
+                        ret = 0
                 finally:
                     val = sys.stdout.getvalue()
                     sys.stdout = old_stdout