Release task should work in "detached HEAD" state

[tschulte]

Some CI servers do checkout branch^{commit} instead of the branch HEAD. I have seen this with Jenkins and Travis.

The release task does not support this, because it always pushes grgit.branch.current.fullName.

My workaround so far is to add git checkout $BRANCH to the CI config. This workaround does not work however, if I want to rebuild a tag using the RebuildVersionStrategy (because e.g. upload to nexus failed and it should be retriggered).

The plugin should allow building in detached HEAD state by auto-detecting if it is run in detached HEAD state.

[ajoberstar]

I don't use detached heads often. In that scenario what should the plugin do? Would it skip the push of the branch and just push the tag? (I'm assuming the push is the only piece you're running into an issue on. Let me know if that's not the case)