filter: `updatebranchcache` during `addchangegroup` instead of after lock

The forced recomputation of the branch cache was introduced by `ee317dbfb9d0`.
Back there, `addchangegroup` did not handle any lock logic.

Later `ee1ed6afac21` introduced lock logic to `addchangegroup`. Its description
does not explain why the `updatebranchcache` call is made outside locking. I
believe that the lock was released there because it fit well with the transaction
release already in the code.

Finally `926a06f7a353` moved all "unlocked" code of `addchangegroup` to an
`repo._afterlock` callback.

I do not think that the call to `updatebranchcache()` requires to be done
outside locking. That may even be a bad idea to do so. Bringing this call back
in the `addchangegroup` function makes the flow simpler and eases the following
up changelog level filtering business.

mercurial/localrepo.py

@@ -2439,10 +2439,10 b' class localrepository(object):'
             tr.close()
 
             if changesets > 0:
+                self.updatebranchcache()
                 def runhooks():
                     # forcefully update the on-disk branch cache
                     self.ui.debug("updating the branch cache\n")
-                    self.updatebranchcache()
                     self.hook("changegroup", node=hex(cl.node(clstart)),
                               source=srctype, url=url)