archive: centralize archive entry writing

archive: centralize archive entry writing

Add the exported function write_archive_entries() to archive.c, which uses
the new ability of read_tree_recursive() to pass a context pointer to its
callback in order to centralize previously duplicated code.

The new callback function write_archive_entry() does the work that every
archiver backend needs to do: loading file contents, entering subdirectories,
handling file attributes, constructing the full path of the entry.  All that
done, it calls the backend specific write_archive_entry_fn_t function.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>