mergemaster fails in single user mode

[Nikolas Britton]

For the sake of clarity and completeness here is a better step by step 
guide for upgrading from src:

0: % su
1: # cvsup -g -L 2 foobar-supfile
2: # cd /usr/src
3: # make buildworld && make buildkernel && make installkernel
4: # reboot
5: Login as root (multi-user mode).
6: Breifly test your new Kernel.
7: # shutdown now
8: Hit the Enter Key.
9: # cd /usr/src
10: # make installworld
11: # reboot
12: login as root (multi-user mode).
13: # mergemaster
14: # shutdown now
15: Hit the Enter Key.
16: # exit