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

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

--- a/mercurial/archival.py	Fri Dec 03 11:40:30 2010 +0900
+++ b/mercurial/archival.py	Tue Dec 07 19:47:53 2010 +1100
@@ -70,6 +70,8 @@
             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