revlog: refactor logic to compute nodemap file in separate function

I will like to use it one more place.

Differential Revision: https://phab.mercurial-scm.org/D9933

mercurial/revlog.py

@@ -448,14 +448,9 b' class revlog(object):'
         self.datafile = datafile or (indexfile[:-2] + b".d")
         self.nodemap_file = None
         if persistentnodemap:
-            if indexfile.endswith(b'.a'):
-                pending_path = indexfile[:-4] + b".n.a"
-                if opener.exists(pending_path):
-                    self.nodemap_file = pending_path
-                else:
-                    self.nodemap_file = indexfile[:-4] + b".n"
-            else:
-                self.nodemap_file = indexfile[:-2] + b".n"
+            self.nodemap_file = nodemaputil.get_nodemap_file(
+                opener, self.indexfile
+            )
 
         self.opener = opener
         #  When True, indexfile is opened with checkambig=True at writing, to

mercurial/revlogutils/nodemap.py

@@ -634,3 +634,14 b' def _find_node(block, node):'
     if isinstance(entry, dict):
         return _find_node(entry, node[1:])
     return entry
+
+
+def get_nodemap_file(opener, indexfile):
+    if indexfile.endswith(b'.a'):
+        pending_path = indexfile[:-4] + b".n.a"
+        if opener.exists(pending_path):
+            return pending_path
+        else:
+            return indexfile[:-4] + b".n"
+    else:
+        return indexfile[:-2] + b".n"