It looks like version 23c76b7 of Raspberry kernel does not exist
anymore:
$ git clone https://github.com/raspberrypi/linux.git
[...]
$ cd linux
$ git co 23c76b7f608e51258c6fca02aebeb5d588583149
fatal: reference is not a tree: 23c76b7f608e51258c6fca02aebeb5d588583149
Replace it by latest version of 3.19 branch.
[Thomas: tweak commit log.]
Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
01e43af20b