Had some problems rebasing a large repository, fatal error because a short SHA-1 ref was ambiguous.
After a quick inspection of git-rebase--interactive.sh, I found that it uses short refs in a few places (git rev-parse --short). Removing that option fixed my problem.
See: https://github.com/git/git/blob/master/git-rebase--interactive.sh#L902
-- Diogo Campos