I have no formal education in computer usage, but I can set up Windows or Linux on a computer or in a VM. Surely your employer thought that a "software developer" could do as much. As for fixing a really borked Windows installation, you just use the installation media to repair it. And as for fixing a borked GNU/Linux installation, you can just install anew to the / partition and leave the /home partition alone. When you get done, you've lost nothing that was on your /home partition.
As for git, I'd need to read the documentation to learn how to use it. But certainly git installs from a .msi or .exe file on Windows ... and since it's in the official software repository of the little known GNU/Linux blend that I use, it's also certainly in all the mainstream GNU/Linux distros' repositories (all you do is point-n-click in the package manager to install git on a GNU/Linux box).
You should have phoned a friend, asked the audience for help, or taken away three of the wrong answers. Surely you know someone who could've help you fix the problem you had. If you had four days to research the problem, you should have had no trouble fixing it before your employment was terminated.
= = = = = = = = = = = =
Don't be too discouraged, though. You may not have known how to fix the problems that were laid upon you. But now you know what you need to learn to succeed next time.
First, learn how to use git: https://git-scm.com/docs.
Next, get a live Linux disk. Perhaps KNOPPIX or PCLinuxOS or openSUSE or one of the 'buntu distros or SystemRescue; it doesn't much matter. Familiarize yourself with whichever one you choose. You can then use the disk to boot the computer (you might benefit from keeping one on a USB flash drive, because it's faster than running GNU/Linux from a live DVD). When you run a computer with a live distro, you can get all sorts of information about the computer, and even repair a borked GNU/Linux (and in some instances, a borked Windows installation). You can certainly get information about the version of a borked Windows installation may be installed on the computer, and then get the installation disk for that version of Windows so that you can use it to repair Windows whether it's XP, Vista, 7, 8.x, or 10.
= = = = = = = = = = = =
If your laptop is still borked, and you don't know how to repair it, you can surely ask about how to do so here in Yahoo! Answers. You can probably do a Google search to get more help than you might get here in Y!A.
= = = = = = = = = = = =
I hope you have more success with your next job. If you're presented with a problem of similar proportions at your next job, try asking questions here in Yahoo! Answers a lot sooner, or do Google searches about the problem(s). Certainly most computer issues are discussed somewhere that will appear as a result in a Google search.