tags: rename "head" to "node" where we don't care

tags: rename "head" to "node" where we don't care

Followup to 5eb4d206202b (tags: extract fnode retrieval into its own
function, 2017-03-28) in which the "for head in head" became "for head
in nodes".

--- a/mercurial/tags.py	Mon Apr 03 08:45:24 2017 -0700
+++ b/mercurial/tags.py	Mon Apr 03 10:01:38 2017 -0700
@@ -380,10 +380,10 @@
     starttime = util.timer()
     fnodescache = hgtagsfnodescache(repo.unfiltered())
     cachefnode = {}
-    for head in reversed(nodes):
-        fnode = fnodescache.getfnode(head)
+    for node in reversed(nodes):
+        fnode = fnodescache.getfnode(node)
         if fnode != nullid:
-            cachefnode[head] = fnode
+            cachefnode[node] = fnode
 
     fnodescache.write()