bundle.h: make "fd" version of read_bundle_header() public

bundle.h: make "fd" version of read_bundle_header() public

Change the parse_bundle_header() function to be non-static, and rename
it to parse_bundle_header_fd(). The parse_bundle_header() function is
already public, and it's a thin wrapper around this function. This
will be used by code that wants to pass a fd to the bundle API.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>