Consider simpler GetPathOfFileAbove syntax

[AArnott]

Could GetPathOfFileAbove and GetDirectoryNameOfFileAbove be made to accept relative paths for their startingDirectory parameter? That way, we can drop the macro that so often is required in the argument.

It would allow this simplification:

-  <Import Project="$([MSBuild]::GetPathOfFileAbove(Directory.Build.props, $(MSBuildThisFileDirectory)..))" Condition=" '$([MSBuild]::GetPathOfFileAbove(Directory.Build.props, $(MSBuildThisFileDirectory)..))' != '' " />
+  <Import Project="$([MSBuild]::GetPathOfFileAbove(Directory.Build.props, ..))" Condition=" '$([MSBuild]::GetPathOfFileAbove(Directory.Build.props, ..))' != '' " />