fetch: trivially refactor assignment to ref_nr

Trivially refactor an assignment to make a subsequent patch
smaller. The "ref_nr" variable is initialized to 0 earlier, just as
"j" is, and "j" is only incremented in that loop, so this change isn't
a logic error.

This change simplifies a subsequent change, which will split the
incrementing of "ref_nr" into two blocks.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -1301,8 +1301,8 @@ static int fetch_one(struct remote *remote, int argc, const char **argv)
 						    argv[i], argv[i]);
 			} else
 				refs[j++] = argv[i];
+			ref_nr++;
 		}
-		ref_nr = j;
 	}
 
 	sigchain_push_common(unlock_pack_on_signal);