attr: Expand macros immediately when encountered.

attr: Expand macros immediately when encountered.

When using macros it is otherwise hard to know whether an
attribute set by the macro should override an already set
attribute. Consider the following .gitattributes file:

[attr]mybinary	binary -ident
*		ident
foo.bin		mybinary
bar.bin		mybinary ident

Without this patch both foo.bin and bar.bin will have
the ident attribute set, which is probably not what
the user expects. With this patch foo.bin will have an
unset ident attribute, while bar.bin will have it set.

Signed-off-by: Henrik Grubbström <grubba@grubba.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>