keepalive: add `__repr__()` to the HTTPConnection class to ease debugging

By default, this just printed the class name and memory address.  By displaying
the address and ports on both sides of the socket, it makes it easier to figure
out what's in the ConnectionManager, and correlate with WireShark traces.

It looks like the two connections mentioned in the previous commit come about
because the LFS POST request to access the blobs opens connection 1, and gets a
401.  Then for some reason, the follow up with credentials opens a new socket,
instead of using the existing one in the pool.  I have no clue why.

This can be seen with something like this in the blobstore:

```
    for h in self.urlopener.handlers:
        if hasattr(h, "close_all"):
            print('open connections on %s in pid %d' % (type(h), os.getpid()))
            for host, conns in h._cm.get_all().items():
                for c in conns:
                    print('connection: %r' % c)
```

mercurial/keepalive.py

@@ -702,6 +702,17 b' class HTTPConnection(httplib.HTTPConnect'
         self.sentbytescount = 0
         self.receivedbytescount = 0
 
+    def __repr__(self):
+        base = super(HTTPConnection, self).__repr__()
+        local = "(unconnected)"
+        s = self.sock
+        if s:
+            try:
+                local = "%s:%d" % s.getsockname()
+            except OSError:
+                pass  # Likely not connected
+        return "<%s: %s <--> %s:%d>" % (base, local, self.host, self.port)
+
 
 #########################################################################
 #####   TEST FUNCTIONS