attr: "binary" attribute should choose built-in "binary" merge driver

The built-in "binary" attribute macro expands to "-diff -text", so
that textual diff is not produced, and the contents will not go
through any CR/LF conversion ever.  During a merge, it should also
choose the "binary" low-level merge driver, but it didn't.

Make it expand to "-diff -merge -text".

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

Documentation/gitattributes.txt

@@ -904,7 +904,7 @@ file at the toplevel (i.e. not in any subdirectory).  The built-in
 macro attribute "binary" is equivalent to:
 
 ------------
-[attr]binary -diff -text
+[attr]binary -diff -merge -text
 ------------
 
 

attr.c

@@ -306,7 +306,7 @@ static void free_attr_elem(struct attr_stack *e)
 }
 
 static const char *builtin_attr[] = {
-	"[attr]binary -diff -text",
+	"[attr]binary -diff -merge -text",
 	NULL,
 };
 

t/t6037-merge-ours-theirs.sh

@@ -54,7 +54,7 @@ test_expect_success 'recursive favouring ours' '
 '
 
 test_expect_success 'binary file with -Xours/-Xtheirs' '
-	echo "file -merge" >.gitattributes &&
+	echo file binary >.gitattributes &&
 
 	git reset --hard master &&
 	git merge -s recursive -X theirs side &&