Git archive and trailing "/" in prefix

Git archive and trailing "/" in prefix

With --prefix=string that does not end with a slash, the top-level entries
are written out with the specified prefix as expected, but no paths in the
directories are added.

Fix this by adding the prefix in write_archive_entry() instead of letting
get_pathspec() and read_tree_recursive() pair; they are designed to only
handle prefixes that are path components.

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