EnableFastComparison succeeds despite changes to postdeploy.sql

- SqlPackage or DacFx Version: latest preview
- .NET Framework (Windows-only) or .NET Core: sqlpackage 170.5.60.2
- Environment (local platform and source/target platforms): Windows

**Steps to Reproduce:**

1. Create a Microsoft.Build.Sql project with a single table and a postdeploy.sql script
2. Publish with EnableFastDeploy and see FastDeploy kick in on second run.
3. Update the post deploy script and rebuild the .dacpac
4. Expected: Full deployment, actual: FasTDeploy kicks in.

Reproduce the behavior with a Microsoft.Build.Sql project containing a table and postdeploy.sql: publish with EnableFastDeploy, modify the script, and rebuild the dacpac. Trace the EnableFastDeploy comparison path first; done means the changed postdeploy.sql causes a full deployment rather than FastDeploy on the second publish.

Written by the indexing model from the issue text.