This PR makes flutter (as measured with flutter test directly in the flutter directory which has no test directory, thus just writing "there's no test directory) startup faster by improving the Windows wrapper script.

When executing flutter in Windows, really it's the bat script flutter.bat that's executed.
This makes sure the user has git installed, there's a git checkout etc.
Specifically it does these things which it doesn't really need to:

• It checks that git is installed and runs a git command. These can be merged because a git command will fail if there is no git.
• It finds powershell even if not used --- and it's only used if updating flutter. This can be postponed until needed.

This PR does exactly that, improving startup time quite significant, see below.

This PR stacks with #111392, #111459 and #111461.

Windows (Google issued with whatever security software)

I.e. this PR alone:

Difference at 95.0% confidence
        -0.764234 +/- 0.030152
        -23.3805% +/- 0.922452%
        (Student's t, pooled s = 0.0320904)

And stacked with #111392, #111459 and #111461:

Difference at 95.0% confidence
        -2.6979 +/- 0.0254303
        -82.5379% +/- 0.777999%
        (Student's t, pooled s = 0.0270652)

A more detailed analysis is available internally at go/FlutterStartupTimeAnalysis.