persistent-nodemap: properly ignore non-existent `.nd` data file

This code was meant to handle the case of a nodemap docket file
pointing to a nodemap data file that doesn’t exist (anymore),
but most likely caused an `UnboundLocalError` exception instead
when `data` was used on the next line without being defined.

This case is theoretically possible with a race condition
between two hg processes, but is hard to reproduce or test:

* Process A reads a docket file and finds a UID in it
  that points to a given data file name.
* Process B decides that this same data file needs compacting.
  It writes a new one with a different UID,
  overwrites the docket file,
  then removes the old data file.
* Only then process A tries to a open a file that doesn’t exist anymore.

Differential Revision: https://phab.mercurial-scm.org/D9533

mercurial/revlogutils/nodemap.py

@@ -58,7 +58,9 b' def persisted_data(revlog):'
             else:
                 data = fd.read(data_length)
     except OSError as e:
-        if e.errno != errno.ENOENT:
+        if e.errno == errno.ENOENT:
+            return None
+        else:
             raise
     if len(data) < data_length:
         return None