v0.2.1 - Performance & Bugfixes

Improve performance and auto generated commit messages when using filters.

See README for a description of the project.

Changes since v0.2.0:

• Include only history of subproject files selected by a given filter in
  the auto-generated commit message
• Improve performance of subproject merges for for source branches with
  lots of files, but restrictive filters
• Include filter expression in auto-generated commit messages

See CHANGELOG for other changes.