git-archive: infer output format from filename when unspecified

git-archive: infer output format from filename when unspecified

A command line

    $ git archive -o my-v2.0.zip v2.0

almost certainly wants the output in zip format, even though it does not
specify any --format option.

When --format is not given, but output filename is, try to infer what
format is requested from the filename extension.  Currently this code only
knows about '.zip'.  When the format is unspecified and the filename does
not tell us, the output will be in 'tar' format as before.

Of course, an explicit --format will not trigger this guesswork.

Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>