archive: read short blobs in archive.c::write_archive_entry()

archive: read short blobs in archive.c::write_archive_entry()

Centralize reading of symlink destinations and the contents of regular
files that are too small to be streamed.  This reduces code duplication
and allows future patches to add support for adding non-tracked files to
archives.  The backends are expected to stream blobs if buffer is NULL.

object_file_to_archive() is only called from archive.c and thus no
longer exported.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>