global: improve const correctness when assigning string constants

We're about to enable `-Wwrite-strings`, which changes the type of
string constants to `const char[]`. Fix various sites where we assign
such constants to non-const variables.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fsck.c

@@ -1231,7 +1231,7 @@ int fsck_object(struct object *obj, void *data, unsigned long size,
 }
 
 int fsck_buffer(const struct object_id *oid, enum object_type type,
-		void *data, unsigned long size,
+		const void *data, unsigned long size,
 		struct fsck_options *options)
 {
 	if (type == OBJ_BLOB)