archive-tar: turn write_tar_entry into blob-writing only

archive-tar: turn write_tar_entry into blob-writing only

Before this patch write_tar_entry() can:

 - write global header
   by write_global_extended_header() calling write_tar_entry with
   with both sha1 and path == NULL

 - write extended header for symlinks, by write_tar_entry() calling
   itself with sha1 != NULL and path == NULL

 - write a normal blob. In this case both sha1 and path are valid.

After this patch, the first two call sites are modified to write the
header without calling write_tar_entry(). The function is now for
writing blobs only. This simplifies handling when write_tar_entry()
learns about large blobs.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>