move encode_in_pack_object_header() to a better place

move encode_in_pack_object_header() to a better place

Commit 1b22b6c897 made duplicated versions of encode_header() into a
common version called encode_in_pack_object_header(). There is however
a better location that sha1_file.c for such a function though, as
sha1_file.c contains nothing related to the creation of packs, and
it is quite populated already.

Also the comment that was moved to the header file should really remain
near the function as it covers implementation details and provides no
information about the actual function interface.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>