bsd-family-tree: Add 2.8BSD relationship to Research 7th edition

In the 2BSD line, the 2.8BSD tapes were the first ones to include a
kernel, both source and a bootable tape. This was an AT&T V7 kernel,
with a number of bug fixes; new features in use at Berkeley; performance
enhancements that were circulating to V7 in the licensee community; and
build system changes. Based on the TUHS archives, it contains none of
the V32 changes, however.

In addition to the source code analysis, Mike Karels relates the story
of how his group lost a customizes to V6 on a PDP-11/40 due to a disk
crash. Since V7 just came out and Bill Jolitz had just brought that up
elsewhere, they replaced their customized V6 with a V7 system, and that
base would eventually become 2.8BSD. (Quarter Century of Unix)

Given both lines of evidence, add a direct line from V7 Unix to 2.8BSD.

Also confirmed that the V6 line to 1BSD and 2BSD was appropriate. 1BSD
and 2BSD included ashell(1) and ex(1). ashell(1) was derived from v6
hell. ex(1) was an enhanced v6 ed. 2.8BSD included process control and
user-land utilities from 4.1BSD

Discussed with: Clem Cole, Diomidis Spinellis (dds)
Differential Revision: https://reviews.freebsd.org/D30883