Correct Content-Type header values for archive downloads.

The content type for both .tar.gz and .tar.bz2 downloads was
application/x-tar, which is correct for .tar files when no
Content-Encoding is present, but is not correct for .tar.gz and .tar.bz2
files unless Content-Encoding is set to gzip or x-bzip2, respectively.

However, setting Content-Encoding causes browsers to undo that encoding
during download, when a .gz or .bz2 file is usually the desired
artifact.  Omitting the Content-Encoding header is preferred to avoid
having browsers uncompress non-render-able files.

Additionally, the Content-Disposition line indicates a final desired
filename with .tar.gz or .tar.bz2 extension which makes providing a
Content-Encoding header inappropriate.

With the current configuration browsers (Chrome and Firefox thus far)
are registering the application/x-tar Content-Type and not .tar
extension and appending that extension, yielding filename.tar.gz.tar as
a final on-disk artifact.  This was originally reported here:
http://stackoverflow.com/questions/3753659

I've changed the .tar.gz and .tar.bz2 Content-Type values to
application/x-gzip and application/x-bzip2, respectively.  Which yields
correctly named download artifacts on Firefox, Chrome, and IE.

mercurial/hgweb/hgweb_mod.py

@@ -281,8 +281,8 b' class hgweb(object):'
                 yield {"type" : i, "extension" : spec[2], "node" : nodeid}
 
     archive_specs = {
-        'bz2': ('application/x-tar', 'tbz2', '.tar.bz2', None),
-        'gz': ('application/x-tar', 'tgz', '.tar.gz', None),
+        'bz2': ('application/x-bzip2', 'tbz2', '.tar.bz2', None),
+        'gz': ('application/x-gzip', 'tgz', '.tar.gz', None),
         'zip': ('application/zip', 'zip', '.zip', None),
         }