vfs: use self.write to write symlink placeholders

The existing write() path is much more robust than the hand-rolled
version that was inlined here.

mercurial/scmutil.py

@@ -327,10 +327,7 b' class vfs(abstractvfs):'
                 raise OSError(err.errno, _('could not symlink to %r: %s') %
                               (src, err.strerror), linkname)
         else:
-            f = self(dst, "w")
-            f.write(src)
-            f.close()
-            self._fixfilemode(dst)
+            self.write(dst, src)
 
     def audit(self, path):
         self.auditor(path)