sane_grep: pass "-a" if grep accepts it

sane_grep: pass "-a" if grep accepts it

Newer versions of GNU grep is reported to be pickier when we feed a
non-ASCII input and break some Porcelain scripts.  As we know we do
not feed random binary file to our own sane_grep wrapper, allow us
to always pass "-a" by setting SANE_TEXT_GREP=-a Makefile variable
to work it around.

Signed-off-by: Junio C Hamano <gitster@pobox.com>