git and the loss of revision numbers

Hi

Disclaimer: I just started to learn git, never used it before.

If I do understand it correctly, the switch from svn to git comes with a loss of continuously increasing revision numbers. Correct? If so I wonder how future security advisories and errata notices will be composed. Will there be a date of the commit besides its hash being reported? 

In the past I could easily judge if there was a need to buildworld or buildkernel: If uname shows a larger revision number than those in advisories or notices.

Question: How may one find out whether to recompile or not in the future?

Thanks and regards,
Michael