Support other tar format configured by git tar.FORMAT.command setting

[KSR-Yasuda]

With checking up git tar.FORMAT_NAME.command setting, support various tar format besides tar and tar.gz.

Now I followed the same way of existing code for tar.gz.
But I suppose it had better output raw tar format in progress, since it does decompression to concatenate tar files.

[KSR-Yasuda]

But I suppose it had better output raw tar format in progress, since it does decompression to concatenate tar files.

Fixed to use raw (i.e. uncompressed) tar format internally for formats except zip.

It does git archive in raw tar files, and call compressing command after concatenation.