[File] Does not correctly detect 2-pass tar.gz files

[Haoqing Chen]

Hi,

It seems like that the file program cannot correctly determine types of 
tar.gz files when they are compressed separately, i.e. by first 
producing a tar file with `tar cf` then passed into gzip to produce the 
final tar.gz file. For such files, using `file -S -iz two-pass.tar.gz` 
will yield application/x-empty instead of the expected application/x-tar.

I done a bit of research and it seems like that it is related to the 
FNAME metadata in the gzipped archive. If the 2nd pass is created with 
`gzip -n` which does not set the FNAME metadata, then file is able to 
correctly output application/x-tar. tar.gz archives created by `tar 
-czf` also seems to lack the FNAME metadata.

Haoqing