Move MockedChangelog to testutils

and allow to specify the content so we can reuse it for other dch
tests.
dch: make automatic adding of new sections more robust

This code that determined if we found a snapshot header was obuscated by
the code that determines the commits to add. Split those and better
document their purpose. Also always return the commit to start from so
we don't need to repeat the logic in the uper levels.
Minor docstring updates
docs: Add recommended branch layout

It'd be nice to have similar branch layouts among packages so recommend
one that worked well so far.
GitRepository: allow to use '..' instead of '...'

The symmetric difference isn't always useful since it includes changes
from both branches. See #680705.
pq: exclude patches from Debian packaging branch

When regenerating the patch queue including patches from the Debian
branch is not useful. Addresses parts of See #680705.

Thanks: Benjamin Cama for the detailed analysis
docs: Refert to gbp in the introduction

since this wraps all commands now.

Thanks: Ross Boylan for pointing out that the current wording is confusing.
docs: remove superfluous at
Use open() instead of file()

since the later doesn't exist in python3
Document changes and release 0.6.3