"Find (git) install" functionality is inconsistent

[StanleyGoldman]

"Find (git) Install" uses a modified path to execute which/where from the Settings tab.
On windows this results in the current selected git path being returned.
On a mac, due to other issues in #659 is unpredictable.

Originally from: #659 (comment)

Also, the "Find Install" function is not working correctly... It does not specify dontSetupGit. As a result on Windows the find task returns the currently configured git installation. I can only wonder what it does on a mac.

Unity/src/UnityExtension/Assets/Editor/GitHub.Unity/UI/GitPathView.cs

Lines 122 to 123 in 977b32e

	new FindExecTask("git", Manager.CancellationToken)
	.Configure(Manager.ProcessManager)

As compared to what happens during mac installation.

Unity/src/GitHub.Api/Installer/GitInstaller.cs

Lines 44 to 47 in 977b32e

	if (!environment.IsWindows)
	{
	startTask = new FindExecTask("git", cancellationToken)
	.Configure(processManager, false, true);