archival: avoid touching deprecated gzip name attribute

archival: avoid touching deprecated gzip name attribute

The existing workaround didn't work when no filename was specified.

If running in a context with warnings enabled and subsecond mtime it gave a
warning:
  DeprecationWarning: use the name attribute

--- a/mercurial/archival.py	Thu Jan 03 21:07:04 2013 +0100
+++ b/mercurial/archival.py	Thu Jan 03 21:07:04 2013 +0100
@@ -74,8 +74,11 @@
         def _write_gzip_header(self):
             self.fileobj.write('\037\213')             # magic header
             self.fileobj.write('\010')                 # compression method
-            # Python 2.6 deprecates self.filename
-            fname = getattr(self, 'name', None) or self.filename
+            # Python 2.6 introduced self.name and deprecated self.filename
+            try:
+                fname = self.name
+            except AttributeError:
+                fname = self.filename
             if fname and fname.endswith('.gz'):
                 fname = fname[:-3]
             flags = 0