shelve: remove useless and incorrect code paths for file access

This patch removes code paths in "shelvedfile.opener()", because:

    - explicit "vfs.mkdir()" invocation is useless

      "vfs.__call__()" for modes other than "read" creates parent
      directory of target file automatically by "util.ensuredirs()".

    - mode checking in "except IOError" code path is useless

      ENOENT occurs only for "read" mode, because target file is
      created forcibly for other modes.

    - there is no explicit "return" statement in the code path for
      "except IOError" if "mode[0] in 'wa'"

      this is incorrect, because None may be returnd unexpectedly,
      even though it seems the EEXIST case in the directory creation
      race for ".hg/shelved" and is very rare.

      this directory creation race is also treated in
      "util.ensuredirs()".

hgext/shelve.py

@@ -66,16 +66,7 class shelvedfile(object):
         except IOError, err:
             if err.errno != errno.ENOENT:
                 raise
-            if mode[0] in 'wa':
-                try:
-                    self.vfs.mkdir()
-                    return self.vfs(self.fname, mode)
-                except IOError, err:
-                    if err.errno != errno.EEXIST:
-                        raise
-            elif mode[0] == 'r':
-                raise util.Abort(_("shelved change '%s' not found") %
-                                 self.name)
+            raise util.Abort(_("shelved change '%s' not found") % self.name)
 
 class shelvedstate(object):
     """Handle persistence during unshelving operations.