Ignore pre-0.7 posh-git prompt

[dahlbyk]

Inspired by #423, here's a relatively simple fix for folks who copied the old prompt function into their $PROFILE. I've tried to make it relatively tolerant of tabs/spaces, CRLF/LF, etc.

[rkeithhill]

If the old profile.example.ps1 defines the prompt function after importing posh-git, then we won't see that prompt function during import.

[dahlbyk]

I don't think there is much we can do about that, short of teaching Write-GitSatus to check prompt. That doesn't feel appropriate.

It may be that this change doesn't fix enough problems for us to bother maintaining.

[rkeithhill]

It may be that this change doesn't fix enough problems for us to bother maintaining.

Yup, although we could add a command to "fix up" the user's profile to remove the old stuff. Or maybe even modify Add-PoshGitToProfile to do that, perhaps with an -Update or -Force parameter.

[dahlbyk]

Ambivalence is fun! Leaning toward leaving it because it's not much code, and it at least does warn me locally that the "wrong" prompt is being used, even though it's not able to actually override that prompt (this is in GitHub Desktop). Doesn't catch everything, but it seems better than not reporting a problematic situation that we can detect.