lei_mirror: drop git <1.8.5 support

lei_mirror: drop git <1.8.5 support

Supporting git <1.8.5 via fetch on non-forkgroup repos would
make auto-GC dangerous, and I want to support auto-GC instead
of relying on the preciousObjects extension.

Since git 1.8.5 is 9 years old at this point, and grokmirror
(used by the only CentOS 7.x user I know of) already relies on
newer git, simplify our code and only fetch into forkgroups.