Hide status symbol when no upstream exists about posh-git-sh

[lyze]

Thank you for your report! This seems like a useful thing.

In 5f42ccd, I fixed a more general issue where there's no indicator for failure from git rev-list, and no upstream is a "failure" mode that trips this. I added an indicator for this, so you can customize this indicator to your liking (perhaps just to a single space?).

The more general question is relevant throughout this script in general (what is worth reporting up to the user vs trying to fall back, swallowing silently, etc.). If there are more requests similar to this, we ought to revisit this for a more principled and unified approach. I think this might be sufficient for now.

Please reopen if you think there's some more to be done for this feature.