Tweaking changelog script to use less bash

The changelog script was complicated by attempting to find things that
should be skipped via the changelog.sh. This was apparently causing
problems for some people.

Fundamentally, this is what is running. The bash parameters provide
the necessary switching.
    git log $range --no-merges --pretty="$pretty_format" --date=short

ATTN: @jcoyne

[skip ci]