store: recommend using `hg debugrebuildfncache` is fncache is corrupted

Differential Revision: https://phab.mercurial-scm.org/D6160

mercurial/store.py

@@ -482,7 +482,9 b' class fncache(object):'
                 pass
 
         if chunk:
-            raise error.Abort(_("fncache does not ends with a newline"))
+            raise error.Abort(_("fncache does not ends with a newline"),
+                              hint=_("use 'hg debugrebuildfncache' to rebuild"
+                                     " the fncache"))
         self._checkentries(fp)
         fp.close()