Timestamp based incrementality does not work with copied files.

This was created from a CLI issue: https://github.com/dotnet/cli/issues/2788
Copying build files does not update their last changed timestamps, therefore MSBuild, which uses timestamp based incrementality, will have false negative incremental checks.

Solutions:
- implement hash based incrementality
- workaround: scripts that copy input build files around should also update the timestamps.

Start with the linked CLI issue (#2788) and investigate how MSBuild performs timestamp-based incremental checks when build files are copied. Compare the proposed hash-based approach with the timestamp-update workaround. Done means copied inputs no longer produce false-negative incremental checks, but the issue does not name files or tests to run.

Written by the indexing model from the issue text.