bundle: add parse_bundle_header() helper function

bundle: add parse_bundle_header() helper function

Move most of the code from read_bundle_header() to parse_bundle_header()
that takes a file descriptor that is already opened for reading, and make
the former responsible only for opening the file and noticing errors.

As a logical consequence of this, is_bundle() helper function can be
implemented as a non-complaining variant of read_bundle_header() that
does not return an open file descriptor, and can be used to tighten
the check used to decide the use of bundle transport in transport_get()
function.

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