Use -LiteralPath to handle paths with []

Fixes #119
Reset Get-LocalOrParentPath() checkin name

commit 72fde6f made $pathToTest use Join-Path and in the process dropped the
fullname from $checkin. On my system, this had the negative of introducing as
PS to my prompt when outside of a git repository. Restoring $checkIn.fullname
leaves the path on the prompt an expected c:\path> instead of c:\pathPS>
maybe fix?
Implement describe style setting and functionality.
Fix branch name when in sync with upstream

Fixes #125
Info about trailing exclamation mark

https://github.com/dahlbyk/posh-git/issues/131
fix ssh-agent start up