Fixed color reset issue

In line [97][1] of the script, the `DefaultForegroundColor` is defined as `\e[0;30m`, which sets the color to black, rather than the color it was using before. In a shell that uses black as its background color, this means everything after the first successful display from posh-git-bash will be nigh-impossible to read.

In my local copy of the script, I've changed it to use `\e[m`, which resets the color scheme. Passing this back upstream to benefit anyone else who uses it.

[1]: https://github.com/lyze/posh-git-bash/blob/4657f74339a234834076ad2646ed365d6215c16d/git-prompt.sh#L97