Before this change we always had a start revision value because
get_latest_tag() returned a SHA if it didn't have a real tag. The way
that SHA is determined leads to not including all changes in the
report about the release, so we don't want to use the value. This
change allows get_latest_tag() to return None and in that case use an
empty starting point for the diff range when computing the release
notes contents.
Change-Id: I7403b0ab2264ef7828c012a943cf093da2951d68
Signed-off-by: Doug Hellmann <doug@doughellmann.com>
f8b4412fae