Why usage function exit with code 129?

From: 徐迪 <xudifsd@gmail.com>
Date: Jan 6, 2012, 07:15 UTC
Message-ID: <CAMocUqSbiO2XC3PUEciyV3HxgqDPW_rTW-hYH4P=C=BFCSrQMQ@mail.gmail.com>

I did like this: $ git add --foo $ echo $? what I got is 129, should this code being used as program terminated due to a signal? Cause it's 128+1. According to this[1].

[1] http://tldp.org/LDP/abs/html/exitcodes.html

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