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 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.