object: introduce get_object_hash macro.

This macro is a temporary change to ease the transition of struct object
to use struct object_id.  It takes an argument of struct object and
returns the object's hash.  Provide this hash next to struct object for
easier conversion.
Signed-off-by: Nbrian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: NJeff King <peff@peff.net>

object.h

@@ -52,6 +52,8 @@ struct object {
 	unsigned char sha1[20];
 };
 
+#define get_object_hash(x) ((x).sha1)
+
 extern const char *typename(unsigned int type);
 extern int type_from_string_gently(const char *str, ssize_t, int gentle);
 #define type_from_string(str) type_from_string_gently(str, -1, 0)