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>

config.mak.uname

@@ -37,6 +37,7 @@ ifeq ($(uname_S),Linux)
 	HAVE_CLOCK_GETTIME = YesPlease
 	HAVE_CLOCK_MONOTONIC = YesPlease
 	HAVE_GETDELIM = YesPlease
+	SANE_TEXT_GREP=-a
 endif
 ifeq ($(uname_S),GNU/kFreeBSD)
 	HAVE_ALLOCA_H = YesPlease