receive-pack.c: die instead of error in case of possible future bug

Discussion on the previous patch revealed we rather want to err on the
safe side. To do so we need to stop receive-pack in case of the possible
future bug when connectivity is not checked on a shallow push.

Also while touching that code we considered that removing the reported
refs may be harmful in some situations. Sound the message more like a
"This Cannot Happen, Please Investigate!" instead of giving advice to
remove refs.

Suggested-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

@@ -1061,9 +1061,7 @@ static void warn_if_skipped_connectivity_check(struct command *commands,
 		}
 	}
 	if (!checked_connectivity)
-		error("BUG: run 'git fsck' for safety.\n"
-		      "If there are errors, try to remove "
-		      "the reported refs above");
+		die("BUG: connectivity check skipped???");
 }
 
 static void execute_commands(struct command *commands,