After installing a Windows System on a System where you already had Linux installed before Windows overwrites the GRUB boot loader. You can no longer choose which system you want to boot at startup. To reinstall GRUB I did the following:

Restart the system from a Linux Live CD (which one is not that important), get a terminal and mount the installed Linux partition you can no longer boot (in my case /dev/sda1):

sudo mount /dev/sda1 /mnt

Mount /dev and /proc into that filesystem:

sudo mount -o bind /dev /mnt/dev
sudo mount -o bind /proc /mnt/proc


chroot into this system:

sudo chroot /mnt

And from there run grub-install:

sudo grub-install /dev/sda

Finish the session:

exit
sudo umount /mnt/dev
sudo umount /mnt/proc
sudo umount /mnt
sudo reboot

GRUB will now come up and you can boot into your old Linux system. You will have to create an entry for Windows in GRUB so you can later also choose to boot into Windows.

If you try to push something to an empty git repository for the first time there’s always this error:

No refs in common and none specified; doing nothing.
Perhaps you should specify a branch such as 'master'.
fatal: The remote end hung up unexpectedly
error: failed to push some refs ...

That happens when you create an empty git repository (“git init –bare –shared“), clone it and add something for the first time.

For this not to happen you simply have to use the following after the first commit:

git push origin head

I always forget that myself and got annoyed a few times in the past because I don’t use git so often. Might help someone else.