changelog.upstream.sh: use rev-list | diff-tree instead of git log

The developer’s Git installation might be set up to use --decorate by
default or to automatically abbreviate commit names.  Use plumbing
commands whose output does not depend on configuration to avoid
gratuitous differences between the changelogs produced on different
machines.

Reported-by: Thorsten Glaser <tg@mirbsd.de>
2 files changed