Problems with git's new safe.directory configuration option

When I try to use an out-of-tree build, running git as the other user fails because the source repository under /home/spwhitton isn't listed in safe.directory (see git-config(1) in recent git). I tried hacking in -c safe.directory="$src" which the documentation leads me to believe would work, but it did not. A workaround is git config --global safe.directory "*" as the other user, but it's not very convenient to have to do that as part of preparing for out-of-tree builds.