If you have synced your tree via rsync in the last days, you may have gotten a stray .git directory in your tree resulting in the following error:

!!! /usr/portage appears to be under revision control (contains .git).
!!! Aborting rsync sync.

You can sync again after removing the directory /usr/portage/.git (replace /usr/portage with your tree location if you've changed its location).

Update (Nov 30): The issue is now fixed on the server and most mirrors. You still need to remove the directory once as described above.