archival: don't set gzip filename header when there's no filename

This mainly affects hgweb, which can generate tar.gz archives without
filenames. Without this change, the header would be set to ".gz",
which can confuse Safari into extracting the file and renaming it to
"gz" when "Open 'safe' files after downloading" is enabled.

file(1) before:

hg-crew-5e51254ad4d4.tar.gz: gzip compressed data, was ".gz", last modified: Thu Dec  2 11:46:20 2010, max compression

after:

hg-crew-5e51254ad4d4.tar.gz: gzip compressed data, last modified: Thu Dec  2 11:46:20 2010, max compression

mercurial/archival.py

@@ -70,6 +70,8 b' class tarit(object):'
             self.fileobj.write('\010')                 # compression method
             # Python 2.6 deprecates self.filename
             fname = getattr(self, 'name', None) or self.filename
+            if fname and fname.endswith('.gz'):
+                fname = fname[:-3]
             flags = 0
             if fname:
                 flags = gzip.FNAME