lts-release-mail: Filter commit messages based on patterns

Instead of excluding 3 last commits before the release tag, exclude commits
based on regex patterns, specifically commits for doc changelog and any
merge commits.

This fixes release email generation for 2.8.15 and a few older releases.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Ib2d6d6188e1d448439bed44be75659b462aaa2a2
1 file changed