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>

git-sh-setup.sh

@@ -168,11 +168,11 @@ git_pager() {
 }
 
 sane_grep () {
-	GREP_OPTIONS= LC_ALL=C grep "$@"
+	GREP_OPTIONS= LC_ALL=C grep @@SANE_TEXT_GREP@@ "$@"
 }
 
 sane_egrep () {
-	GREP_OPTIONS= LC_ALL=C egrep "$@"
+	GREP_OPTIONS= LC_ALL=C egrep @@SANE_TEXT_GREP@@ "$@"
 }
 
 is_bare_repository () {