tags: silence hgtagsfnodes reading failures

tryread() doesn't handle "is a directory" errors and presumably
others. We might not want to globally swallow such tryread errors, so
we replace with our own try/except handling.

An upcoming test will use directories as a portable stand-in for
various bizarre circumstances that cache read/write code should be
robust to.

mercurial/tags.py

@@ -432,7 +432,10 b' class hgtagsfnodescache(object):'
 
         self._raw = array('c')
 
-        data = repo.vfs.tryread(_fnodescachefile)
+        try:
+            data = repo.vfs.read(_fnodescachefile)
+        except (OSError, IOError):
+            data = ""
         self._raw.fromstring(data)
 
         # The end state of self._raw is an array that is of the exact length