largefiles: don't query the dirstate for key None

lfutil.splitstandin(f) can be None, and we query the dirstate for that without
checking if it is. This will cause problems with the upcoming move to critbit-
based dicts, since they only support strings as keys.

hgext/largefiles/overrides.py

@@ -374,10 +374,13 b' def overridemanifestmerge(origfn, repo, '
         f, m, args, msg = action
 
         choices = (_('&Largefile'), _('&Normal file'))
-        if m == "g" and lfutil.splitstandin(f) in p1 and f in p2:
+
+        splitstandin = lfutil.splitstandin(f)
+        if (m == "g" and splitstandin is not None and
+            splitstandin in p1 and f in p2):
             # Case 1: normal file in the working copy, largefile in
             # the second parent
-            lfile = lfutil.splitstandin(f)
+            lfile = splitstandin
             standin = f
             msg = _('%s has been turned into a largefile\n'
                     'use (l)argefile or keep as (n)ormal file?') % lfile