revlog: use radix tree also for matching keys shorter than 4 hex digits

I don't know what the reason for the 4-digit limit was, and I can't
think of any real disadvantages of using the radix tree also when the
requested minimum length is short. This speeds up `hg log -T
'{shortest(node,1)}\n'` from 2m16s to 4.5s by making that not fall
back to pure code.

Differential Revision: https://phab.mercurial-scm.org/D3453

mercurial/cext/revlog.c

@@ -1251,7 +1251,7 b' static PyObject *index_partialmatch(inde'
 	if (!PyArg_ParseTuple(args, PY23("s#", "y#"), &node, &nodelen))
 		return NULL;
 
-	if (nodelen < 4) {
+	if (nodelen < 1) {
 		PyErr_SetString(PyExc_ValueError, "key too short");
 		return NULL;
 	}