Look for and process sem-ver pseudo headers in git

At the moment careful changelog review is needed by humans to
determine the next version number. Semantic versioning means that all
we need to know to get a reasonable default next version (and a lower
bound on the next version) is a record of bugfix/deprecation/feature
work and api-breaking commits. In this patch we start scanning for
such headers from the git history to remove this burden from
developers/project release managers. Higher versions can of course
be used either via pre-versioning or by tagging the desired version.

implements: blueprint pbr-semver
sem-ver: feature
Change-Id: Id5e8cd723d5186d1bd8c01599eae8933e6f7ea6d
Merge "Look for and process sem-ver pseudo headers in git"