Handle rebases with no new commits

[ruuda]

If rebasing results in no new commit (because all commits were present on the branch already), Hoff will still push the (already existing) commit to testing, and wait for a build result to come in. This will not happen, because origin/testing is equal to origin/master at that point. If HEAD is equal to master after a rebase, the pull request should be considered as merged.

[ruuda]

This no longer happens with the interactive rebase; now the rebase fails. Hoff should report a specific error, rather than "failed to rebase".