archive: honor tar.umask even for pax headers

archive: honor tar.umask even for pax headers

git archive's tar format uses extended pax headers to encode metadata
into the archive.  Most tar implementations correctly treat these as
metadata, but some that do not understand the pax format extract these
as files instead.  Apply the tar.umask setting to these entries to
prevent tampering by other users.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>