git-archive: ignore prefix when checking file attribute

git-archive: ignore prefix when checking file attribute

Ulrik Sverdrup noticed that git-archive doesn't correctly apply the attribute
export-subst when the option --prefix is given, too.

When it checked if a file has the attribute turned on, git-archive would try
to look up the full path -- including the prefix -- in .gitattributes.  That's
wrong, as the prefix doesn't need to have any relation to any existing
directories, tracked or not.

This patch makes git-archive ignore the prefix when looking up if value of the
attribute export-subst for a file.

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