PR Baseline Update Tool is Incorrectly Deleting Files

[ellahathaway]

See dotnet/dotnet#329.

The PR baseline update tool is incorrectly deleting files (eg in the referenced PR above, all of the baseline files have been deleted or moved).

The sdk diff files were moved from test/Microsoft.DotNet.SourceBuild.Tests/assets/SdkContentTests/MsftToSbSdkAssemblyVersions.diff -> test/Microsoft.DotNet.SourceBuild.Tests/assets/MsftToSbSdkAssemblyVersions.diff, so I suspect that the issue is related to where the baseline update tool is targetting the update (eg assets instead of /assets/SdkContentTests)