revlog: remove micro-optimization for looking up only nullid

index_find_node() would call nt_find() before initializing the node
tree. nt_find() would then return -2 unless the requested revision was
the null revision. I can't imagine what scenario that is optimizing
for, and doing the initialization earlier makes the code simpler and
easier to follow, so that's what this patch does.

Differential Revision: https://phab.mercurial-scm.org/D4027

mercurial/cext/revlog.c

@@ -994,9 +994,6 b' static int nt_find(indexObject *self, co'
 	if (nodelen == 20 && node[0] == '\0' && memcmp(node, nullid, 20) == 0)
 		return -1;
 
-	if (self->nt == NULL)
-		return -2;
-
 	if (hex)
 		maxlevel = nodelen > 40 ? 40 : (int)nodelen;
 	else
@@ -1133,14 +1130,14 b' static int index_find_node(indexObject *'
 {
 	int rev;
 
+	if (nt_init(self) == -1)
+		return -3;
+
 	self->ntlookups++;
 	rev = nt_find(self, node, nodelen, 0);
 	if (rev >= -1)
 		return rev;
 
-	if (nt_init(self) == -1)
-		return -3;
-
 	/*
 	 * For the first handful of lookups, we scan the entire index,
 	 * and cache only the matching nodes. This optimizes for cases