Backport patches for improving gitattributes handling

This merge request backports patches from the following upstream merge requests:

• https://github.com/agx/git-buildpackage/pull/74
• https://github.com/agx/git-buildpackage/pull/75
• https://github.com/agx/git-buildpackage/pull/77
• https://github.com/agx/git-buildpackage/pull/78