dirs: speed up by storing number of direct children per dir

The Python version of the dirs type stores only the number of direct
children associated with each directory. That means that while adding
a directory, it only has to walk backwards until it runs into a
directory that is already in its map. The C version walks all the way
to the top-most directory. By copying the Python version's clever
trick to the C code, we can speed it up quite a bit.

On the Firefox repo, perfdirs now runs in 0.031390, from 0.056518
before the undoing Sid's optimization in the previous change, and
0.061835 before previous his optimization. More practically, it speeds
up 'hg status nonexistent' on the Firefox repo from 0.176s to 0.155s.

It's unclear why the C version did not have the same cleverness
implemented from the start, especially given that they were both
written by the same person (Bryan O'Sullivan) very close in time:

  856960173630 (scmutil: add a dirs class, 2013-04-10)
  02ee846b246a (scmutil: rewrite dirs in C, use if available, 2013-04-10)

mercurial/dirs.c

@@ -69,7 +69,7 b' static int _addpath(PyObject *dirs, PyOb'
 		val = PyDict_GetItem(dirs, key);
 		if (val != NULL) {
 			PyInt_AS_LONG(val) += 1;
-			continue;
+			break;
 		}
 
 		/* Force Python to not reuse a small shared int. */
@@ -114,9 +114,11 b' static int _delpath(PyObject *dirs, PyOb'
 			goto bail;
 		}
 
-		if (--PyInt_AS_LONG(val) <= 0 &&
-		    PyDict_DelItem(dirs, key) == -1)
-			goto bail;
+		if (--PyInt_AS_LONG(val) <= 0) {
+			if (PyDict_DelItem(dirs, key) == -1)
+				goto bail;
+		} else
+			break;
 		Py_CLEAR(key);
 	}
 	ret = 0;