Revisiting mergemaster truncating mergemaster.mtree

Hi Doug,

just recreated the old "mergemaster -p truncates mtree". The machine in 
question is 7.2-p4, to be upgraded to stable/8. Luckily for me, I copied the 
mtree file to .bak before doing this (justified paranoia :).
history snippet:
  280  sudo cp /var/db/mergemaster.mtree /var/db/mergemaster.mtree.bak
  282  sudo -E make installkernel
  283  sudo -E mergemaster -p
  284  sudo -E make installworld
  285  sudo -E mergemaster -iUF
       ^^ ^C there, "no mtree database found"

Result:
-rw-------   1 root      wheel          0 Nov 11 10:35 mergemaster.mtree
-rw-------   1 root      wheel     165681 Nov 11 10:32 mergemaster.mtree.bak

Environment:
% sudo -E env|grep -Ev '^(SSH|TERM|STY|WINDOW|SUDO)'
OLDPWD=/usr/src
SHELL=/usr/local/bin/zsh
_=/usr/local/bin/sudo
MAKEOBJDIRPREFIX=/data/obj
PWD=/usr/src
SAVEHIST=5000
HISTSIZE=500
HISTFILE=/home/mel/.zsh_history
HELPDIR=/usr/local/lib/zsh/help
EDITOR=vim
PAGER=less -i -s -g
LESS=-cigex3M
MAIL=/var/spool/mail/mel
SHLVL=2
USER=root
LOGNAME=root
HOME=/home/mel
PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/games:/usr/local/sbin:/usr/local/bin:/home/mel/bin
CCACHE_DIR=/data/ccache/mel
HTTP_PROXY=http://squid/
FTP_PASSIVE_MODE=YES
BLOCKSIZE=K
USERNAME=root

No /etc/mergemaster* or ~/.mergemasterrc.

The machine was installed from 7.2 CD and freebsd-update'd to -p4. Let me know 
if there's anything else I can provide to help track this down, but I have to 
proceed with upgrade for this specific machine.
FWIW, I've been unable to reproduce this lately on stable/7 or stable/8 
machines, but since it was never identified I figured you may be interested to 
know what codepath does this, so it doesn't reoccur.
-- 
Mel