Add option to manage distribution fields for non snapshot mode.

Add option to manage distribution fields for non snapshot mode.

The snapshot mode fails to merge two "debian/changelog" entries if the
distribution is not "UNRELEASED".

* tests/11_test_dch_main.py: test the new --distribution option to
  dch.main(). Add only meaningful tests, i.e. options alone and combined
  with --release and --snapshot.

* git-dch (fixup_section): Rename as it's used to fix header in addition
  to trailer, this require a new named parameter "options".
  (main): Add version_group option "-D" and "--distribution" to
  specify the distribution name.
  Add version_group option "--force-distribution" to force the provided
  distribution to be used, used by "dch".
  Rename fixup_trailer() to fixup_section() and add options as parameters.

* docs/manpages/git-dch.sgml: Update documentation for new options.

Closes: #646684