bug#25458: 25.1; tar mode does not handle compressed archives without specific extensions

[Francesco Potortì]

>>>But I think the question is -- do we want to support this?  I'm not
>>>quite sure -- it would be pretty unusual for a mode to do something like
>>>this, and it's not clear what the semantics should be.  That is, if
>>>we're saving the tar buffer afterwards, should it be compressed or not?
>>>Either option would surprise somebody.
>>
>> Not really.  If I read a compressed file, Emacs uncompresses it when
>> loading it to a buffer, then you can edit it and save it.  Compression
>> and decompression are transparent.  I use this feature quite often,
>> and so I am led to expect it of compressed archives too.  If it was
>> compressed to begin with, it should be saved compressed too.
>
>But the file name is "foo.tar" -- there'd be no way to tell Emacs that
>the user wants to save that uncompressed.

In fact, if the file was compressed, Emacs should save it compressed with the 
same method.

>You can load bar.gz into a buffer, and write out bar, and Emacs won't
>compress that.

- I load bar.gz
- Edit it
- C-x C-s saves it with the same name, compressed with the same method

Same with foo.tar.gz

It should be the same with foo.tar, if foo.tar was compressed

And no, there would be no way to tell Emacs to save foo.tar uncompressed.  I do 
not think that it would be important to be able to save it uncompressed.