localrepo: introduce "prepushoutgoinghooks" to extend outgoing check easily

This patch introduces "prepushoutgoinghooks" to extend outgoing check
before pushing changesets to remote easily.

This chooses the function returning "util.hooks" instead of the one to
be overridden.

The latter may cause problems silently, if one of overriders forgets
(or fails) to execute a kind of "super(xxx, self).overridden(...)". In
the other hand, the former can ensure that all registered functions
are invoked, unless one of them raises an exception.

mercurial/exchange.py

@@ -103,6 +103,9 b' def push(repo, remote, force=False, revs'
         try:
             _pushdiscovery(pushop)
             if _pushcheckoutgoing(pushop):
+                pushop.repo.prepushoutgoinghooks(pushop.repo,
+                                                 pushop.remote,
+                                                 pushop.outgoing)
                 _pushchangeset(pushop)
             _pushcomputecommonheads(pushop)
             _pushsyncphase(pushop)

mercurial/localrepo.py

@@ -1696,6 +1696,13 b' class localrepository(object):'
         """
         pass
 
+    @unfilteredpropertycache
+    def prepushoutgoinghooks(self):
+        """Return util.hooks consists of "(repo, remote, outgoing)"
+        functions, which are called before pushing changesets.
+        """
+        return util.hooks()
+
     def push(self, remote, force=False, revs=None, newbranch=False):
         return exchange.push(self, remote, force, revs, newbranch)