bundle: allowing to read from an unseekable fd

bundle: allowing to read from an unseekable fd

We wished that "git bundle" to eventually learn to read from a network
socket which is not seekable. The current code opens with fopen(), reads
the file halfway and run ftell(), and reopens the same file with open()
and seeks, to skip the header.

This patch by itself does not reach that goal yet, but I think it is a
right step in that direction.

Signed-off-by: Junio C Hamano <gitster@pobox.com>