disable lookup for GitHub's version of Git

[shiftkey]

👋 GitHub Desktop team member dropping in. We had report from someone who was seeing our app launch every few minutes, trying to open a weird path: desktop/desktop#2962

Repro steps:

• on Windows
• have classic GitHub for Windows installed
• install new GitHub Desktop
• open a private Git repository in Code
• force a pull to trigger retrieving credentials

Actual behaviour:

GitHub Desktop is launched, trying to open a folder that doesn't look like a folder:

Expected behaviour:

No launching UI

What's Happening

The classic Git Shell had system-level config value to make itself act as a credential helper:

[credential]
	helper = !github --credentials

This was fine because we put github.exe first on the PATH when you launched Git Shell and only then, but we never added ourselves to the user's PATH for historical reasons (not wanting to interfere with existing Git installations mostly). Time passed, things changed, and with the new Desktop people wanted us to bring back the github command line for doing things like adding repositories to the app.

This time around we didn't have a Git Shell in-the-box, so we appended the location of github.bat to PATH so that it'd be available in whatever shell you like.

Now we have different problems, because github.bat doesn't act as a credential helper, and the classic github.exe will call github.bat and never itself because your PATH is setup that way.

So I'd like to just 🔥 this lookup code and encourage people to setup Git for Windows.

[joaomoreno]

Makes sense, thanks @shiftkey!