Pick default archive format based on filename suffix

Pick default archive format based on filename suffix

Introduce a setFilename() method for ArchiveCommand so callers can
specify the intended filename of the produced archive.  If the
filename ends with .tar, the format will default to tar; if .zip, zip;
if .tar.gz, gzip-compressed tar; and so on.

This doesn't affect "jgit archive" because it doesn't support the
--output=<file> option yet.  A later patch might do that.

Change-Id: Ic0236a70f7aa7f2271c3ef11083b21ee986b4df5