attr.c: simplify macroexpand_one()

attr.c: simplify macroexpand_one()

The double-loop wants to do an early return immediately when one
matching macro is found.  Eliminate the extra variable 'a' used for
that purpose and rewrite the "assign the found item to 'a' to make
it non-NULL and force the loop(s) to terminate" with a direct return
from there.

Signed-off-by: Junio C Hamano <redacted>
Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Brandon Williams <redacted>
Signed-off-by: Junio C Hamano <redacted>