rename read_replace_refs to check_replace_refs

The semantics of this flag was changed in commit

    e1111cef23 inline lookup_replace_object() calls

but wasn't renamed at the time to minimize code churn.  Rename it now,
and add a comment explaining its use.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

upload-pack.c

@@ -796,7 +796,7 @@ int main(int argc, char **argv)
 
 	packet_trace_identity("upload-pack");
 	git_extract_argv0_path(argv[0]);
-	read_replace_refs = 0;
+	check_replace_refs = 0;
 
 	for (i = 1; i < argc; i++) {
 		char *arg = argv[i];