If the upstream sources are already on a separate branch, things are pretty simple. You can either rename that branch to the default upstream-branch name upstream with:

  git branch -m theupstream-branch upstream

or you can tell gbp buildpackage the name of the branch to use as upstream-branch:

cat <<EOF > .git/gbp.conf
[DEFAULT]
# this is the upstream-branch:
upstream-branch=theupstream-branch
EOF

If you then use gbp import-orig to import new upstream sources, they will from now on end up on theupstream-branch and merged to the debian-branch.

If you don't have an upstream branch but started your repository with only the upstream sources (not the Debian™ patch), you can simply branch from that point. So use gitk or git-log to locate the commit-id of that commit and create the upstream branch from there, e.g.:

    git branch upstream $(git log --format='%H' | tail -1)

The important thing here is that the COMMIT_ID specifies a point on the master branch that carried only the upstream sources and not the Debian™ modifications. The above example assumes that this was the first commit to that repository.

In order to fix this you can prepend the upstream sources as a single commit to your tree using Git™'s grafts. Afterwards you can simply create a branch as explained above and gbp import-orig will work as expected.

Alternatively, if you are only importing source from original tarballs (for instance when converting from a Subversion repository where the mergeWithUpstream was set for svn-buildpackage), you can create an empty upstream branch with the following commands:

  git checkout --orphan upstream
  git rm -rf .
  git commit --allow-empty -m 'Initial upstream branch.'
  git checkout -f master
  # When not using 3.0 (quilt) with the default --merge-mode=auto
  git merge --allow-unrelated-histories upstream