Consider writing our own GitHub detection task

[idg10]

We currently depend on Microsoft.Build.Tasks.Git to discover things about the git repo. Unfortunately, it turns out to be impossible to use this in a way that does report a warning if you're not in a Git repo.

So we had to add a horrible hack to detect whether we think we're in a Git repo before using that component. At some stage we might want to look at the following options

• asking nicely whether we could add something to Microsoft.Build.Tasks.Git to suppress that warning
• writing our own custom task DLL so we don't depend on that external component, and can just do exactly what we want

[MikeEvansLarah]

As part of this we could also consider warning people that they are not working in a Git initialized repo (e.g. "you have been working in here for 4 days, should you be using Git?")