Add support for "jgit archive --output=<filename>"

Add support for "jgit archive --output=<filename>"

If the --format option is not given and the output filename is,
then infer the format from that filename.  Otherwise match
"git archive" by defaulting to tar (this is a change from the
existing "jgit archive" default behavior, which was to default to
zip).

Change-Id: I5806bc48a403d05e4cfc3c180b82b33ad7cfae7f