archive: refactor file extension format-guessing

Git-archive will guess a format from the output filename if
no format is explicitly given.  The current function just
hardcodes "zip" to the zip format, and leaves everything
else NULL (which will default to tar). Since we are about
to add user-specified formats, we need to be more flexible.
The new rule is "if a filename ends with a dot and the name
of a format, it matches that format". For the existing "tar"
and "zip" formats, this is identical to the current
behavior. For new user-specified formats, this will do what
the user expects if they name their formats appropriately.

Because we will eventually start matching arbitrary
user-specified extensions that may include dots, the strrchr
search for the final dot is not sufficient. We need to do an
actual suffix match with each extension.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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