Add RevisionTimeStamp as an MSBuild property

It could be useful to know what a commit's timestamp was, in addition to the commit hash/id.

This will allow downstream tooling to use it as appropriate. For example, some basic deployment tools only check file timestamps, so it could be interesting to modify files last write time to the commit timestamp, in order to get deterministic outputs, but still compatible with tools that only use timestamps.

I suggest the timestamp should be in ISO8601 format, possibly with an option to force a specific timezone (commit's original timezone or current machine's local timezone, or UTC)