The following function is duplicated:

  encode_header

Move this function to sha1_file.c and rename it 'encode_in_pack_object_header',
as suggested by Junio C Hamano
Signed-off-by: NMichael Lukashov <michael.lukashov@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>